Remove unnecessary(?) warnings_not_errors for vcl KDE code

Change-Id: I6e342aed2832d7a6ab029889a019829a349b2dce
Reviewed-on: https://gerrit.libreoffice.org/8775
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>