We've been sending all events to clients immediately for quite some time
now, so this is only really impacting the Clutter scene graph, not
clients anymore.
That makes this behavior a somewhat unnecessary optimization (it was
useful at the time it was added, but it's not anymore), which will only
make our lives harder when we actually expect an event to be queued
(eg. in tests), so remove it.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2697>
fe01e423a3