Remove redundant button clicked handler in QQuickMessageBox

Change c286b4fccb2d83fcc01b21913b95c5e4f21f2982 added the
QQuickAbstractMessageDialog::click() handler, which made the
QQuickMessageDialog::clicked() handler redundant.

Change-Id: Ifd770a3c67b35fd9a4929fc55b9e5298c36fb960
Task-number: QTBUG-35933
Reviewed-by: Liang Qi <liang.qi@digia.com>