Do not force loading of Qt Creator's ios plugin

The plugin isn't marked as experimental anymore since Qt Creator 3.1.0.

Task-number: QTBUG-41835
Change-Id: Ie6c0dfc15638dbd1a2920c49403cea6f00fcdf0c
Reviewed-by: Jani Heikkinen <jani.heikkinen@digia.com>
Reviewed-by: Iikka Eklund <iikka.eklund@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>