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.

configure

@@ -673,7 +673,6 @@ base_machine
 have_tunables
 build_pt_chown
 build_nscd
-build_obsolete_nsl
 link_obsolete_rpc
 libc_cv_static_nss_crypt
 libc_cv_nss_crypt
@@ -786,7 +785,6 @@ enable_experimental_malloc
 enable_crypt
 enable_nss_crypt
 enable_obsolete_rpc
-enable_obsolete_nsl
 enable_systemtap
 enable_build_nscd
 enable_nscd
@@ -1459,8 +1457,6 @@ Optional Features:
   --enable-nss-crypt      enable libcrypt to use nss
   --enable-obsolete-rpc   build and install the obsolete RPC code for
                           link-time usage
-  --enable-obsolete-nsl   build and install the obsolete libnsl library and
-                          depending NSS modules
   --enable-systemtap      enable systemtap static probe points [default=no]
   --disable-build-nscd    disable building and installing the nscd daemon
   --disable-nscd          library functions will not contact the nscd daemon
@@ -3654,20 +3650,6 @@ if test "$link_obsolete_rpc" = yes; then
 
 fi
 
-# Check whether --enable-obsolete-nsl was given.
-if test "${enable_obsolete_nsl+set}" = set; then :
-  enableval=$enable_obsolete_nsl; build_obsolete_nsl=$enableval
-else
-  build_obsolete_nsl=no
-fi
-
-
-
-if test "$build_obsolete_nsl" = yes; then
-  $as_echo "#define LINK_OBSOLETE_NSL 1" >>confdefs.h
-
-fi
-
 # Check whether --enable-systemtap was given.
 if test "${enable_systemtap+set}" = set; then :
   enableval=$enable_systemtap; systemtap=$enableval
@@ -3785,7 +3767,7 @@ main ()
 {
 
 #ifndef __CET__
-#error no CET compiler support
+# error no CET compiler support
 #endif
   ;
   return 0;