selftests/futex: don't redefine .PHONY targets (all, clean)

The .PHONY targets "all" and "clean"  are both already defined in the
file that is included in the very next line:

    ../lib.mk.

Remove this duplicate code.

Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>