Remove pointless project property forwardings in pluginspec.qbs.

They might have been necessary in ancient times, but nowadays project
properties can be accessed in prepare scripts.

Change-Id: I209b3c7d0caba7c2f4fc2dda80e57b16d8255fe0
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>