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.

config/routes.rb

@@ -164,7 +164,6 @@ Rails.application.routes.draw do
   resources :note_versions, :only => [:index, :show]
   resource :note_previews, only: [:create, :show]
   resource :password_reset, only: [:create, :show]
-  resources :pixiv_ugoira_frame_data, only: [:index]
   resources :pools do
     member do
       put :revert