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/factories/pixiv_ugoira_frame_data.rb

@@ -1,12 +0,0 @@
-FactoryBot.define do
-  factory(:pixiv_ugoira_frame_data) do
-    post
-    content_type { "image/jpeg" }
-    data do
-      [
-        { "file" => "000000.jpg", "delay" => 200 },
-        { "file" => "000001.jpg", "delay" => 200 },
-      ]
-    end
-  end
-end