remove unused `#optional_parts`

This method was copied from journey at https://github.com/rails/rails/commit/56fee39c392788314c44a575b3fd66e16a50c8b5#diff-2cfaf53c860732fea8689d6f2002594bR78.
`grep -nr 'optional_parts' .`

actionpack/lib/action_dispatch/journey/route.rb

@@ -68,10 +68,6 @@ def format(path_options)
         @path_formatter.evaluate path_options
       end
 
-      def optional_parts
-        path.optional_names.map(&:to_sym)
-      end
-
       def required_parts
         @required_parts ||= path.required_names.map(&:to_sym)
       end