Defang X509_REQ_{s,g}et_extension_nids()

Defang X509_REQ_{s,g}et_extension_nids()

These fiddle with unprotected global state, so aren't thread safe and
of course there was no good reason to have this API in the first place.
Nothing uses it, so it becomes a noop and will be removed in the next
major bump.

ok jsing