selftests:prctl: Fix make clean override warning

Remove clean target in Makefile to fix the following warning
and use the one in common lib.mk

Makefile:14: warning: overriding recipe for target 'clean'
../lib.mk:160: warning: ignoring old recipe for target 'clean'
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/prctl/Makefile

@@ -10,7 +10,5 @@ all: $(TEST_PROGS)
 
 include ../lib.mk
 
-clean:
-	rm -fr $(TEST_PROGS)
 endif
 endif