vcl: remove System as friend of OutputDevice

Change-Id: I5ca8c8b1a4cc4c031e5966887b41af766df9fe68
Reviewed-on: https://gerrit.libreoffice.org/13036
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>