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 acb527929d0c2b3bb0798472c42ddb3203729708 ("Move
non-deprecated RPC-related functions from sunrpc to inet") in
combination with commit 5500cdba4018ddbda7909bc7f4f9718610b43cf0
("Remove --enable-obsolete-rpc configure flag").
