meson: drop ln --relative check

The coreutils release that included this was in 2012. Since then,
systemd requires a newer meson, mostly uses install_symlink, and
so on. Additionally, where it does use this feature, it uses the
'-r' short flag. There are implementations of 'ln' that do have
support for '-r' but no longopts. Things will still work on those
but this check will fail.

meson.build

@@ -659,10 +659,6 @@ foreach prog : progs
         conf.set_quoted(name, path)
 endforeach
 
-if run_command(ln, '--relative', '--help', check : false).returncode() != 0
-        error('ln does not support --relative (added in coreutils 8.16)')
-endif
-
 #####################################################################
 
 gperf_test_format = '''