QtSql: remove deprecated signal 'notification(QString)'

It was deprecated in Qt5 and the replacement signal with three arguments
can be used instead.

Change-Id: I6d0db8d9fa9bea2039c548e32bc39a8173403c3a
Reviewed-by: Vitaly Fanaskov <vitaly.fanaskov@qt.io>
Reviewed-by: Andy Shaw <andy.shaw@qt.io>