Remove no-op `Relation::WhereClauseFactory` constant

That is to prevent the "uninitialized constant" error when loading the
data marshalized by the previous version of Rails (Rails 6.0), it is not
needed for the future version of Rails.

activerecord/lib/active_record/relation/query_methods.rb

@@ -1605,11 +1605,4 @@ module ActiveRecord
         end
       end
   end
-
-  class Relation # :nodoc:
-    # No-op WhereClauseFactory to work Mashal.load(File.read("legacy_relation.dump")).
-    # TODO: Remove the class once Rails 6.1 has released.
-    class WhereClauseFactory # :nodoc:
-    end
-  end
 end