C++: Skip generating the AddDesc function

[acozzette]

We are changing the protobuf compiler and renaming some of the generated C++ functions. This causes RPCZ to break since RPCZ's generated code calls the protobuf generated AddDesc function. Here is an example snippet showing what this looks like:

void rpcz_protobuf_AddDesc_search_2eproto() {
  static bool already_here = false;
  if (already_here) return;
  already_here = true;
  GOOGLE_PROTOBUF_VERIFY_VERSION;

  ...
}

However, it appears that the AddDesc function generated by RPCZ is never called and is not exposed in any header files, so this commit just deletes the RPCZ code responsible for generating it.

[acozzette]

@thesamet Let me know if this looks ok; I keep thinking I must be missing something but I couldn't find anything that depended on this code I removed.

[thesamet]

LGTM - thanks for the PR.

[acozzette]

Thanks, @thesamet!