This is an old relic from when ClutterStageView was being added, and
tests were somewhat prepared to be able to test the "X11 style" of
things, with the nested backend some how managing to emulate that.
Lets drop that stuff, it isn't used by the test suite, and isn't useful
anyway; if we want to test X11 configurations, we should use the actual
X11 backend, which didn't make use of this anyway.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2619>
fc8a4afc45