models: remove creator_id from artists, notes, and pools.

Remove the creator_id field from artists, notes, and pools. The
creator_id wasn't otherwise used and was inconsistent with the
artist/note/pool history in some cases, especially for old artists.

test/factories/note.rb

@@ -1,6 +1,5 @@
 FactoryBot.define do
   factory(:note) do
-    creator
     post
     x { 1 }
     y { 1 }