packaging: fix qt.conf for ios tools

Qt 5.3.1 does not set the profix only based on the use of QGuiApplication
but actually checks if the app is a bundle, so the Prefix hack in the
qt.conf for the ios tools is not needed anymore.

Task-number: QTCREATORBUG-12393
Change-Id: I6a228d83370bc62929ffd056b9b3051e4562485e
Reviewed-by: Jani Heikkinen <jani.heikkinen@digia.com>
Reviewed-by: Kai Koehne <kai.koehne@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>