Remove broken reference to `Download`
Remove broken reference to `Download`

As part of open sourcing our Ruby typechecker, Sorbet (https://sorbet.run/) we're using it to check large open source Ruby projects. When I ran it on your project, it reported the error:

```
./test/unit/rubygem_test.rb:799: Unable to resolve constant Download http://go/e/5002
     799 |        1.times { Download.incr(@rubygem.name, @version.full_name) }
                            ^^^^^^^^
    ./app/models/gem_download.rb:1: Did you mean: GemDownload?
     1 |class GemDownload < ApplicationRecord
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

It looks like this constant indeed doesn't exist in your codebase (that I can tell, please correct me if I'm wrong). This specific code will never run because there are no `should` blocks in it so I think that is why it wasn't caught by your runtime testing.
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