Avoid double deletion of QQuickWindow

It's uncertain why 4fc0df58

 added
the call to sendPostedEvents(0, QEvent::DeferredDelete) but now we can
see that it easily results in the destructor calling itself, and
therefore double-deleting its own d_ptr.  removePostedEvents seems
safer to ensure that the window cannot be doubly deleted, in spite
of the qdoc warning that "You should never need to call this function."

Task-number: QTBUG-33436
Change-Id: I4873ebe179dde551407eba1f6baac5f03ca7f177
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>