Delete test for returning nil when requested variant not yet ready

I tried swapping out the test definition for the ImageData model
instead, but the behaviour is different for ImageData - the
CarrierWave relative path is returned instead of the URL (as in the
`test "returns store path when the model has no assets, although it
should (still uploading or error has occurred)" do` test.

I'm not entirely clear why the behaviour of ImageData is different
to AttachmentData, but given attachments now only ever have the
'original' variant, I doubt this test is needed anymore. I'm also
not sure if there's a bit of corresponding logic in the AttachmentData
model that can be deleted (or the AssetManagerStorage model, which
is, after all, the thing we're supposed to be testing in this file).
Couldn't see any code simplification opportunities, so settling for
a deleted test.

Here's the code I attempted to swap it with:

```
  test "returns nil when the model has assets but the requested variant is not yet available" do
    model = build(:image_data)
    model.assets = model.assets - [model.assets.last]
    model.save!
    model.reload

    assert_nil model.file.url("s216".to_sym) # what would have been the 'last' asset
  end
```

test/unit/lib/whitehall/asset_manager_storage_test.rb

@@ -53,17 +53,6 @@ class Whitehall::AssetManagerStorage::FileTest < ActiveSupport::TestCase
     assert_equal "http://assets-host/media/asset_manager_id/sample.docx", model.file.url
   end
 
-  # TODO: commented out to get the tests passing.
-  # We should better understand how to rewrite this test and whether it is still valuable.
-  #
-  # test "returns nil when the model has assets but the requested variant is not available" do
-  #   model = build(:attachment_data_with_asset, attachable: build(:draft_edition, id: 1))
-  #   model.save!
-  #   model.reload
-
-  #   assert_nil model.file.url(:thumbnail)
-  # end
-
   test "returns store path when the model has no assets, although it should (still uploading or error has occurred)" do
     model = build(:attachment_data_with_no_assets, attachable: build(:draft_edition, id: 1))
     model.save!