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>