In the past, barries were added to the window management X11 display
instance window table, and then special cased when iterating over the
list.
Since then, barriers, which are really part of the backend, has stopped
being added to the window hash table, instead being managed by the
backend. Lets clean up the left-over special casing that is no longer
needed.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2442>