Drop undefined doc config path
Drop undefined doc config path

html_static_path doesn't exist and the following warning is present in
logs:

```
WARNING: html_static_path entry '_static' does not exist
```

These changes drop this config.

Change-Id: I5ca35d1658c8734dd74d1c4dd16a8516f16c39b6

openstack/ceilometer,openstack/ceilometer