Move <rpc/netdb.h> from sunrpc to inet

Restore <rpc/netdb.h> as an installed header. Delete the dummy header
resolv/rpc/netdb.h because inet is not an optional glibc component
(so its <rpc/netdb.h> is always available).

Fixes commit acb52792 ("Move
non-deprecated RPC-related functions from sunrpc to inet") in
combination with commit 5500cdba
("Remove --enable-obsolete-rpc configure flag").