tools/Makefile: remove cgroup target

The tools/cgroup directory no longer contains a Makefile.  This patch
updates the top-level tools/Makefile to remove references to building and
installing cgroup components.  This change reflects the current structure
of the tools directory and fixes the build failure when building tools in
the top-level directory.

linux/tools$ make cgroup
  DESCEND cgroup
make[1]: *** No targets specified and no makefile found.  Stop.
make: *** [Makefile:73: cgroup] Error 2

Link: https://lkml.kernel.org/r/20240315012249.439639-1-liucong2@kylinos.cn
Signed-off-by: Cong Liu <liucong2@kylinos.cn>
Acked-by: Stanislav Fomichev <sdf@google.com>
Reviewed-by: Dmitry Rokosov <ddrokosov@salutedevices.com>
Cc: Cong Liu <liucong2@kylinos.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>