make the plugin template not create a _dependencies.pri file

it's pointless and counterproductive for 3rd party plugins.

Task-number: QTCREATORBUG-9192
Change-Id: Id9a59e44fa37d00ff5bd68b4240517f0d014c7a2
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>