Doc: Removed extra "\group events" page in Qt GUI.

The group page is in Qt Core already.

Task-number: QTBUG-32580
Change-Id: If9c4b40bad96c138dd8fe98e71378f73269c66e4
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>