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.

app/controllers/artists_controller.rb

@@ -44,7 +44,7 @@ class ArtistsController < ApplicationController
   end
 
   def create
-    @artist = Artist.create(artist_params.merge(creator: CurrentUser.user))
+    @artist = Artist.create(artist_params)
     respond_with(@artist)
   end