Client: Remove mWaitingForUpdateDelivery

https://codereview.qt-project.org/c/qt/qtwayland/+/393273 got accepted, so that part of the mutex fix series can go in. Hopefully makes https://bugs.kde.org/show_bug.cgi?id=449163 less likely on its own already.

Currently, mWaitingForUpdateDelivery is shared between the main thread
(doHandleFrameCallback()) and the frame callback event thread
(handleFrameCallback()), however the access to it is not synchronized
between both threads. On the other hand, QWaylandWindow
already ensures not to create a frame callback if there's already one
pending.

This change removes mWaitingForUpdateDelivery flag because it should be
already covered by mWaitingForFrameCallback and to remove unsynchronized
shared state between threads.

Change-Id: I0e5a25d18d1e66c4d7683e7e972330c4d7cbbf38
Reviewed-by: David Edmundson <davidedmundson@kde.org>
(cherry picked from commit feb1a5c207c13d0bf87c0d8ad039279dbf8cee9e)