core/events: Don't update the cursor renderer on motion under X11
Doing so was unnecessary and seemed to cause feedback to Xorg resulting in heavy stuttering and random freezes.
We do however keep the call to meta_cursor_tracker_invalidate_position
so as to not undo the intention of 9bdf4b35.
Suggested by Carlos Garnacho.
Fixes: gnome-shell#5861 (closed) and https://launchpad.net/bugs/1989582