drm/dp_mst: Remove register_connector callback

Now drm_dp_mst_topology_cbs.register_connector callback is not getting
used anymore hence remove it.

Signed-off-by: Pankaj Bharadiya <[e-mail]>
Suggested-by: Emil Velikov <[e-mail]>
Suggested-by: Lyude Paul <[e-mail]>
Signed-off-by: Lyude Paul <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Lyude Paul <[e-mail]>

include/drm/drm_dp_mst_helper.h

···
 struct drm_dp_mst_topology_cbs {
 	/* create a connector for a port */
 	struct drm_connector *(*add_connector)(struct drm_dp_mst_topology_mgr *mgr, struct drm_dp_mst_port *port, const char *path);
-	void (*register_connector)(struct drm_connector *connector);
 	void (*destroy_connector)(struct drm_dp_mst_topology_mgr *mgr,
 				  struct drm_connector *connector);
 };