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/artist.rb

@@ -1,6 +1,5 @@
 FactoryBot.define do
   factory(:artist) do
-    creator
     name { rand(1_000_000).to_s }
     is_active { true }
   end

test/factories/note.rb

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

test/factories/pool.rb

@@ -1,6 +1,5 @@
 FactoryBot.define do
   factory(:pool) do
-    creator
     name {"pool_" + rand(100..1000099).to_s}
     description {FFaker::Lorem.sentences.join(" ")}
   end