runtime: drop unused C type reflection code

In particular, drop __go_type_descriptors_equal, which is no longer
used, and will be made obsolete by CL 179598.

Change-Id: Id6a0380faf40f41fb73b0807e23ffebf60573011
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/179858
Reviewed-by: Cherry Zhang <cherryyz@google.com>