Remove legacy test about asset variant URL including original ID

There are now no attachment types (other than images and their
variants) that result in anything other than the 'original' file
being uploaded to Asset Manager.

Whilst I could have changed this test to test a model that DOES
have variants (i.e. ImageData), with code something like what I've
put below, this behaviour is already covered by the "constructs
the url of the file using the assets root, media, asset_manager_id
and filename" test in this file, so I've deleted it instead.

```
    asset_path = "path/to/asset.png"
    model = ImageData.new
    model.id = 1
    model.assets << build(:asset, asset_manager_id: @asset_manager_id, variant: Asset.variants[:s216], filename: "asset_216.png")

    file = Whitehall::AssetManagerStorage::File.new(asset_path, model, "s216")

    assert_equal "http://assets-host/media/#{@asset_manager_id}/asset_216.png", file.url
```

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 look at this again with fresh eyes - are we testing the right thing?
-  #
-  # test "returns file url using asset_manager_id when the model has an asset variant" do
-  #   model = build(:attachment_data, attachable: build(:draft_edition, id: 1))
-  #   model.save!
-  #   model.reload
-
-  #   assert_equal "http://assets-host/media/asset_manager_id_thumbnail/thumbnail_greenpaper.pdf.png", model.file.url(:thumbnail)
-  # end
-
   # TODO: commented out to get the tests passing.
   # We should better understand how to rewrite this test and whether it is still valuable.
   #