Client: Remove windows from keyboard focus list when destroyed

This fixes the undefined behavior in tst_WaylandClient::touchDrag and mouseDrag

Note: The test still fails if run twice in a row, but it appears to be
deterministic.

Task-number: QTBUG-56187
Change-Id: Ib45d82224f004d1324f2ce4d6b7df05ee36c04f5
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>
(cherry picked from commit 0049240a)
Reviewed-by: Johan Helsing <johan.helsing@qt.io>