qxl: Remove unused prototype

qxl_crtc_set_from_monitors_config() is defined in qxl_drv.h but never
implemented.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>
Message-id: 20161108091209.25568-3-cfergeau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>