clone3: clone3_cap_checkpoint_restore: remove unused MAX_PID_NS_LEVEL macro

The macro MAX_PID_NS_LEVEL is never referenced in the code, so remove it.

Signed-off-by: Ba Jing <bajing@cmss.chinamobile.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>