Fix assert on deleting installed custom URL scheme handler

When the QObject::destroyed signal is emitted the inherited class parts
have already been destroyed and thus it is no longer a
QWebEngineUrlSchemeHandler and qobject_cast will return 0, which is
asserted against.

Change-Id: I7130c60a26088067930499a30e0081ed297a92d9
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>