drm/dp_mst: Remove register_connector callback

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 <pankaj.laxminarayan.bharadiya@intel.com>
Suggested-by: Emil Velikov <emil.velikov@collabora.com>
Suggested-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200307083023.76498-4-pankaj.laxminarayan.bharadiya@intel.com
Reviewed-by: Lyude Paul <lyude@redhat.com>