selftests: clone3: remove duplicate defines

Remove duplicate defines which are already included in kernel headers.
MAX_PID_NS_LEVEL macro is used inside kernel only. It isn't exposed to
userspace. So it is never defined in test application. Remove #ifndef in
this case.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>