kernel/swap: remove redundant ARG_UNUSED from do_swap()

The `lock` arg is used multiple times in the function, making the
`ARG_UNUSED(lock);` redundant, remove it.

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