Delete layout_builder entries in tempstore when relationship is deleted

We have to do this tempstore keyspecific

'.sites_group:' . $group->id()

I ran into a fatal in sites_path_alias when I tried deleting the relationship while being masqueraded - but it seems to be unrelated to these changes. I'll create an issue there.

Oh see: https://git.erdfisch.de/contrib/sites/-/merge_requests/87/diffs?commit_i...