Remove HTTP mappings from status.h

The mappings are a combination of useless, inaccurate, and harmful.
Leave them out until they are helpful and correct.

Fundamental issues:
 - If in need of mappings, a user would need HTTP -> gRPC code mappings.
   But the docs don't provide that since the mappings are not 1:1.
 - There _is no_ gRPC -> HTTP code mapping taking place in gRPC. This
   may change in the future, but until then the docs are misleading.

But even given those were resolved, there are additional issues. Two
obvious examples:
 - 501 Not Implemented is refering to HTTP methods such as GET and POST
   not being implement. That should not be used for gRPC methods not
   found (UNIMPLEMENTED).
 - 404 Not Found is for when the resource is not found. In gRPC, the
   resource is the gRPC method, not the parameters (NOT_FOUND). If there
   is a REST conversion layer on top of gRPC, then 404 would be
   appropriate, but that just shows that the mapping can not be
   canonical.