db: drop normalized_url column from artist_urls.

Unused since 0d480eb832.

db/migrate/20220921022408_drop_normalized_url_from_artist_urls.rb

@@ -0,0 +1,5 @@
+class DropNormalizedURLFromArtistURLs < ActiveRecord::Migration[7.0]
+  def change
+    remove_column :artist_urls, :normalized_url, :text
+  end
+end