Remove redirect for failed image requests
Remove redirect for failed image requests

The assets served by the HmrcAssetsController are a fixed set of files
that are put in place manually[1]. They only include files ending in
`.zip`, `.xml` and `.bak`. On this basis the call to `image?` in this
controller would always return `false` and can be removed.

[1] The current set of assets are:

```
/data/uploads/whitehall/clean/uploaded/
└── hmrc
    ├── paye-test
    ├── payetools-linux.zip
    ├── payetools-osx-intel.zip
    ├── payetools-osx-ppc.zip
    ├── payetools-rti-13.0.13071.40-linux.zip
    ├── payetools-rti-13.0.13071.41-win-assistive.zip
    ├── payetools-rti-13.0.13071.41-win.zip
    ├── payetools-rti-13.0.13071.42-osx.zip
    ├── payetools-rti-13.0.13099.28-linux.zip
    ├── payetools-rti-13.0.13099.29-osx.zip
    ├── payetools-rti-13.0.13099.30-win-assistive.zip
    ├── payetools-rti-13.0.13099.30-win.zip
    ├── payetools-rti-13.0.19999.179-linux.zip
    ├── payetools-rti-13.0.19999.180-osx.zip
    ├── payetools-rti-13.0.19999.278-win.zip
    ├── payetools-rti-13.1.13122.101-linux.zip
    ├── payetools-rti-13.1.13122.102-win-assistive.zip
    ├── payetools-rti-13.1.13122.102-win.zip
    ├── payetools-rti-13.1.13122.103-osx.zip
    ├── payetools-rti-13.1.13137.110-linux.zip
    ├── payetools-rti-13.1.13137.111-osx.zip
    ├── payetools-rti-13.1.13137.112-win-assistive.zip
    ├── payetools-rti-13.1.13137.112-win.zip
    ├── payetools-rti-13.2.13231.64-osx.zip
    ├── payetools-rti-13.2.13232.64-linux.zip
    ├── payetools-rti-13.2.13232.64-win-assistive.zip
    ├── payetools-rti-13.2.13232.64-win.zip
    ├── payetools-rti-14.0.14063.106-linux.zip
    ├── payetools-rti-14.0.14063.106-osx.zip
    ├── payetools-rti-14.0.14063.106-win-assistive.zip
    ├── payetools-rti-14.0.14063.106-win.zip
    ├── payetools-rti-14.0.14098.109-osx.zip
    ├── payetools-rti-14.1.14168.197-linux.zip
    ├── payetools-rti-14.1.14168.197-osx.zip
    ├── payetools-rti-14.1.14168.197-win-assistive.zip
    ├── payetools-rti-14.1.14168.197-win.zip
    ├── payetools-rti-14.2.14233.55-linux.zip
    ├── payetools-rti-14.2.14233.55-osx.zip
    ├── payetools-rti-14.2.14233.55-win-assistive.zip
    ├── payetools-rti-14.2.14233.55-win.zip
    ├── payetools-rti-14.2.14330.88-linux.zip
    ├── payetools-rti-14.2.14330.88-osx.zip
    ├── payetools-rti-14.2.14330.88-win-assistive.zip
    ├── payetools-rti-14.2.14330.88-win.zip
    ├── payetools-rti-15.0.15048.300-linux.zip
    ├── payetools-rti-15.0.15048.300-osx.zip
    ├── payetools-rti-15.0.15048.300-win-assistive.zip
    ├── payetools-rti-15.0.15048.300-win.zip
    ├── payetools-rti-15.1.15162.94-linux.zip
    ├── payetools-rti-15.1.15162.94-osx.zip
    ├── payetools-rti-15.1.15162.94-win-assistive.zip
    ├── payetools-rti-15.1.15162.94-win.zip
    ├── payetools-rti-16.0.16076.450-linux.zip
    ├── payetools-rti-16.0.16076.450-osx.zip
    ├── payetools-rti-16.0.16076.450-win-assistive.zip
    ├── payetools-rti-16.0.16076.450-win.zip
    ├── payetools-rti-16.1.16125.489-linux.zip
    ├── payetools-rti-16.1.16125.489-osx.zip
    ├── payetools-rti-16.1.16125.489-win-assistive.zip
    ├── payetools-rti-16.1.16125.489-win.zip
    ├── payetools-rti-17.0.17068.356-linux.zip
    ├── payetools-rti-17.0.17068.356-osx.zip
    ├── payetools-rti-17.0.17068.356-win-assistive.zip
    ├── payetools-rti-17.0.17068.356-win.zip
    ├── payetools-rti-17.1.17118.423-linux.zip
    ├── payetools-rti-17.1.17118.423-osx.zip
    ├── payetools-rti-17.1.17118.423-win-assistive.zip
    ├── payetools-rti-17.1.17118.423-win.zip
    ├── payetools-rti-17.2.17125.433-linux.zip
    ├── payetools-rti-17.2.17125.433-osx.zip
    ├── payetools-rti-17.2.17125.433-win-assistive.zip
    ├── payetools-rti-17.2.17125.433-win.zip
    ├── payetools-rti-17.3.17151.465-linux.zip
    ├── payetools-rti-17.3.17151.465-osx.zip
    ├── payetools-rti-17.3.17151.465-win-assistive.zip
    ├── payetools-rti-17.3.17151.465-win.zip
    ├── payetools-rti-17.4.17200.477-linux.zip
    ├── payetools-rti-17.4.17200.477-osx.zip
    ├── payetools-rti-17.4.17200.477-win-assistive.zip
    ├── payetools-rti-17.4.17200.477-win.zip
    ├── payetools-rti-patch-13.1.13122.101-linux.zip
    ├── payetools-rti-patch-13.1.13122.102-win.zip
    ├── payetools-rti-patch-13.1.13122.103-osx.zip
    ├── payetools-rti-patch-13.1.13137.110-linux.zip
    ├── payetools-rti-patch-13.1.13137.111-osx.zip
    ├── payetools-rti-patch-13.1.13137.112-win.zip
    ├── payetools-rti-patch-13.2.13231.64-osx.zip
    ├── payetools-rti-patch-13.2.13232.64-linux.zip
    ├── payetools-rti-patch-13.2.13232.64-win.zip
    ├── payetools-rti-patch-14.0.14063.106-linux.zip
    ├── payetools-rti-patch-14.0.14063.106-osx.zip
    ├── payetools-rti-patch-14.0.14063.106-win.zip
    ├── payetools-rti-patch-14.0.14098.109-osx.zip
    ├── payetools-rti-patch-14.1.14168.197-linux.zip
    ├── payetools-rti-patch-14.1.14168.197-osx.zip
    ├── payetools-rti-patch-14.1.14168.197-win.zip
    ├── payetools-rti-patch-14.2.14233.55-linux.zip
    ├── payetools-rti-patch-14.2.14233.55-osx.zip
    ├── payetools-rti-patch-14.2.14233.55-win.zip
    ├── payetools-rti-patch-14.2.14330.88-linux.zip
    ├── payetools-rti-patch-14.2.14330.88-osx.zip
    ├── payetools-rti-patch-14.2.14330.88-win.zip
    ├── payetools-rti-patch-15.0.15048.300-linux.zip
    ├── payetools-rti-patch-15.0.15048.300-osx.zip
    ├── payetools-rti-patch-15.0.15048.300-win.zip
    ├── payetools-rti-patch-15.1.15162.94-linux.zip
    ├── payetools-rti-patch-15.1.15162.94-osx.zip
    ├── payetools-rti-patch-15.1.15162.94-win.zip
    ├── payetools-rti-patch-16.0.16076.450-linux.zip
    ├── payetools-rti-patch-16.0.16076.450-osx.zip
    ├── payetools-rti-patch-16.0.16076.450-win.zip
    ├── payetools-rti-patch-16.1.16125.489-linux.zip
    ├── payetools-rti-patch-16.1.16125.489-osx.zip
    ├── payetools-rti-patch-16.1.16125.489-win.zip
    ├── payetools-rti-patch-17.0.17068.356-linux.zip
    ├── payetools-rti-patch-17.0.17068.356-osx.zip
    ├── payetools-rti-patch-17.0.17068.356-win.zip
    ├── payetools-rti-patch-17.1.17118.423-linux.zip
    ├── payetools-rti-patch-17.1.17118.423-osx.zip
    ├── payetools-rti-patch-17.1.17118.423-win.zip
    ├── payetools-rti-patch-17.2.17125.433-linux.zip
    ├── payetools-rti-patch-17.2.17125.433-osx.zip
    ├── payetools-rti-patch-17.2.17125.433-win.zip
    ├── payetools-rti-patch-17.3.17151.465-linux.zip
    ├── payetools-rti-patch-17.3.17151.465-osx.zip
    ├── payetools-rti-patch-17.3.17151.465-win.zip
    ├── payetools-rti-patch-17.4.17200.477-linux.zip
    ├── payetools-rti-patch-17.4.17200.477-osx.zip
    ├── payetools-rti-patch-17.4.17200.477-win.zip
    ├── payetools-windows.zip
    ├── realtimepayetools-linux.zip
    ├── realtimepayetools-osx-intel.zip
    ├── realtimepayetools-update-v14.xml
    ├── realtimepayetools-update-v15.xml
    ├── realtimepayetools-update-v15.xml.bak
    ├── realtimepayetools-update-v16.bak
    ├── realtimepayetools-update-v16.xml
    ├── realtimepayetools-update-v16.xml.bak
    ├── realtimepayetools-update-v17.xml
    ├── realtimepayetools-update.xml
    ├── realtimepayetools-windows.zip
    ├── rti-Core-patch.xml
    ├── test-realtimepayetools-update-v14.xml
    ├── test- realtimepayetools-update-v15.xml
    ├── test-realtimepayetools-update-v15.xml
    ├── test- realtimepayetools-update-v16.xml
    ├── test-realtimepayetools-update-v17.xml
    ├── test-realtimepayetools-update.xml
    └── XMLFiles.zip
```

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall