drm/dp_mst: Remove drm_dp_mst_topology_cbs.destroy_connector

Now that we've removed the last user of this callback, get rid of it and
drm_dp_destroy_connector().

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200331205740.135525-5-lyude@redhat.com


Reviewed-by: Alex Deucher <alexdeucher@gmail.com>