drm: remove unused DRM_MINOR_UNASSIGNED

This constant is unused, remove it.
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

include/drm/drmP.h

@@ -1008,7 +1008,6 @@ struct drm_driver {
 	struct list_head legacy_dev_list;
 };
 
-#define DRM_MINOR_UNASSIGNED 0
 #define DRM_MINOR_LEGACY 1
 #define DRM_MINOR_CONTROL 2
 #define DRM_MINOR_RENDER 3