Remove Secure RPC DES authentication

Following the earlier removal of keyserv, none of this functionality
works since it requires keyserv.

Remove the relevant symbols from libc's Symbol.map. Leave compatibility
symbols for existing applications, but since the functions don't work
without keyserv, stub them out to return an error.

Remove some private symbols that were only used by keyserv; these don't
get compatibility symbols.

Remove the documentation for the old functions.

Remove rpc.ypupdated since it requires DES authentication.

Reviewed by: manu, des, emaste
Differential Revision: https://reviews.freebsd.org/D50442