vm_swapout: Remove a special case from vm_swapout_map_deactivate_pages()

John points out that this probably should have been removed in commit
472888018ce, which removed a special case where we'd set desired=0 if
the target process has P_INMEM clear. It's not obvious to me that the
desired=0 case can't arise by setting an RSS limit to 0, but I'm not
sure why we'd try to go the extra mile in that case anyway.

Reported by: jhb
Reviewed by: kib
MFC after: 2 weeks
Fixes: 472888018ce1 ("proc: Remove kernel stack swapping support, part 6")
Differential Revision: https://reviews.freebsd.org/D56140