Remove an unused classmethod tags_for(). Anyways it would not work because its trying to filter on a GenericForeignKey.

Removed an unused classmethod `tags_for()` of TaggedItemBase. This method is never used  because GenericTaggedItemBase also has a `tags_for()` defined and whenever `tags_for()` is used on a TaggedItem, it will use the one it inherits from GenericTaggedItemBase and not the one it inherits from TaggedItemBase.

Moreover, the filter condition used in the removed tags_for() was trying to filter on variable `content_object` which is a GenericForeignKey, so anyways this filter condition would never have worked.