Remove unused Artist#legacy_api_hash method (#3206).

Added in 0e0d83c7 but never used. app/views/legacy/artists.{json,xml}.erb
is what *would* use this, but neither does.

app/models/artist.rb

@@ -540,18 +540,6 @@ class Artist < ApplicationRecord
     def hidden_attributes
       super + [:other_names_index]
     end
-
-    def legacy_api_hash
-      return {
-        :id => id,
-        :name => name,
-        :other_names => other_names,
-        :group_name => group_name,
-        :urls => artist_urls.map {|x| x.url},
-        :is_active => is_active?,
-        :updater_id => 0
-      }
-    end
   end
 
   include UrlMethods