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

test/unit/artist_test.rb

@@ -524,7 +524,7 @@ class ArtistTest < ActiveSupport::TestCase
     context "#new_with_defaults" do
       should "fetch the defaults from the given source" do
         source = "https://i.pximg.net/img-original/img/2018/01/28/23/56/50/67014762_p0.jpg"
-        artist = Artist.new_with_defaults(source: source, creator: create(:user))
+        artist = Artist.new_with_defaults(source: source)
 
         assert_equal("niceandcool", artist.name)
         assert_equal("nice_and_cool", artist.other_names_string)