artist urls: stop using normalized_url.

Stop the last remaining uses of the `artist_urls.normalized_url` column.
It's already no longer used by the artist finder. The only remaining
uses were by API users. Those users should use the `url` column instead.

test/unit/artist_test.rb

@@ -134,7 +134,7 @@ class ArtistTest < ActiveSupport::TestCase
 
     should "allow fixing invalid urls" do
       artist = FactoryBot.build(:artist)
-      artist.urls << FactoryBot.build(:artist_url, url: "www.example.com", normalized_url: "www.example.com")
+      artist.urls << FactoryBot.build(:artist_url, url: "www.example.com")
       artist.save(validate: false)
 
       artist.update(url_string: "http://www.example.com")