Remove --enable-obsolete-nsl configure flag

this means that *always* libnsl is only built as shared library for
backward compatibility and the NSS modules libnss_nis and libnss_nisplus
are not built at all, libnsl's headers aren't installed.

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

Replacement implementations based on TIRPC, which additionally support
IPv6, are available from <https://github.com/thkukuk/>.

This change does not affect libnss_compat which does not depended
on libnsl since 2.27 and thus can be used without NIS.

libnsl code depends on Sun RPC, e.g. on --enable-obsolete-rpc (installed
libnsl headers use installed Sun RPC headers), which will be removed in
the following commit.

nis/libnsl.h

@@ -23,9 +23,6 @@
 #define NSS_FLAG_ADJUNCT_AS_SHADOW	8
 
 
-/* Get current set of default flags.  */
-extern int _nsl_default_nss (void);
-
 /* Set up everything for a call to __do_niscall3.  */
 extern nis_error __prepare_niscall (const_nis_name name, directory_obj **dirp,
 				    dir_binding *bptrp, unsigned int flags);