sources: simplify artist finder url normalization.

Get rid of `normalized_for_artist_finder?` and `normalizable_for_artist_finder?`.
This was legacy bullshit that was originally designed to avoid API calls
when saving artist entries containing old Pixiv direct image urls that
had already been normalized, or that couldn't be normalized because they
were bad id.

Nowadays we store profile urls in artist entries instead of direct image
urls, so we don't normally need to do any API calls to normalize the
profile url. Strategies should take care to avoid triggering API calls
inside `profile_url` when possible.

app/logical/sources/strategies/null.rb

@@ -13,18 +13,6 @@ module Sources
         image_url
       end
 
-      def normalized_for_artist_finder?
-        true
-      end
-
-      def normalizable_for_artist_finder?
-        false
-      end
-
-      def normalize_for_artist_finder
-        url
-      end
-
       def normalize_for_source
         case url
         when %r{\Ahttps?://(?:d3j5vwomefv46c|dn3pm25xmtlyu)\.cloudfront\.net/photos/large/(\d+)\.}i