drm/qxl: drop redundant code

Not needed, qxl_io_destroy_primary() does that for us.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20210511104522.2694803-2-kraxel@redhat.com