drm/nouveau: Remove bogus cleanup in nv50_mstm_add_connector()

Trying to destroy the connector using mstc->connector.funcs->destroy()
if connector initialization fails is wrong: there is no possible
codepath in nv50_mstc_new where nv50_mstm_add_connector() would return
<0 and mstc would be non-NULL.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ben Skeggs <bskeggs@redhat.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@redhat.com>
Cc: Jerry Zuo <Jerry.Zuo@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Juston Li <juston.li@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190111005343.17443-13-lyude@redhat.com