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.

app/models/media_metadata.rb

@@ -28,4 +28,8 @@ class MediaMetadata < ApplicationRecord
   def metadata
     ExifTool::Metadata.new(self[:metadata])
   end
+
+  def frame_delays
+    metadata["Ugoira:FrameDelays"].to_a
+  end
 end