ugoira: remove the PixivUgoiraFrameData model.

Remove the last remaining uses of the PixivUgoiraFrameData model. As of
32bfb8407, Ugoira frame data is now stored in the MediaMetadata model,
under the `Ugoira:FrameDelays` EXIF field.

The pixiv_ugoira_frame_data table still exists, but it can be removed
after this commit is deployed.

Fixes #5264: Error when replacing with ugoira.

test/unit/sources/pixiv_test.rb

@@ -46,8 +46,7 @@ module Sources
         should "capture the frame data" do
           media_file = @site.download_file!(@site.image_urls.sole)
 
-          assert_equal(2, media_file.frame_data.size)
-          assert_equal([{"file" => "000000.jpg", "delay" => 125}, {"file" => "000001.jpg", "delay" => 125}], media_file.frame_data)
+          assert_equal([125, 125], media_file.frame_delays)
         end
       end