kernel: remove `CONFIG_MP_NUM_CPUS`

`CONFIG_MP_NUM_CPUS` has been deprecated for more than 2
releases, it's time to remove it.

Updated all usage of `CONFIG_MP_NUM_CPUS` to
`CONFIG_MP_MAX_NUM_CPUS`

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>