Tagging.remove_unused_tags should gracefully handle already deleted tags

[irphilli]

Together with https://github.com/salsify/goldiloader, I noticed that the order of how tags/taggings are deleted can cause the remove_unused_tags callback to be called multiple times. This is normally fine, however, the callback performs a reload on a tag, and that tag may have already been deleted. remove_unused_tags should gracefully handle already deleted tags.