glibc: Don't install sln

It's a 835 KiB utility to create symlinks that (AFAIK) is not used
anywhere.  It also has no use case in NixOS.

pkgs/development/libraries/glibc/2.18/builder.sh

@@ -49,6 +49,9 @@ postInstall() {
     # This file, that should not remain in the glibc derivation,
     # may have not been created during the preInstall
     rm -f $out/lib/libgcc_s.so.1
+
+    # Get rid of more unnecessary stuff.
+    rm -rf $out/var $out/sbin/sln
 }
 
 genericBuild