Remove --enable-obsolete-rpc configure flag

Sun RPC was removed from glibc. This includes rpcgen program, librpcsvc,
and Sun RPC headers. Also test for bug #20790 was removed
(test for rpcgen).

Backward compatibility for old programs is kept only for architectures
and ABIs that have been added in or before version 2.28.

libtirpc is mature enough, librpcsvc and rpcgen are provided in
rpcsvc-proto project.

NOTE: libnsl code depends on Sun RPC (installed libnsl headers use
installed Sun RPC headers), thus --enable-obsolete-rpc was a dependency
for --enable-obsolete-nsl (removed in a previous commit).

The arc ABI list file has to be updated because the port was added
with the sunrpc symbols

Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>