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>

tools/testing/selftests/futex/Makefile

@@ -3,8 +3,6 @@ SUBDIRS := functional
 
 TEST_PROGS := run.sh
 
-.PHONY: all clean
-
 include ../lib.mk
 
 all: