Reduce the usage of QT_PREPEND_NAMESPACE in the Cocoa platform plugin

This is no longer required as QT_USE_NAMESPACE is used most of the time

Task-number: QTBUG-23946

Change-Id: Ia6225260531adbf34ec1846805b8fe1d66dbaef8
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@digia.com>