amd64: Remove support for "nooptions SMP"

It does not appear to get much, if any, testing, and doesn't seem to be
worth the maintenance overhead. Virtually all amd64 hardware has
multiple cores. The CPU and memory usage overhead of the SMP option in
single-vCPU VMs is quite marginal and not worth maintaining.

Reviewed by: alc (pmap.c), kib
Differential Revision: https://reviews.freebsd.org/D51403
Differential Revision: https://reviews.freebsd.org/D51345