Remove sysctl 'kern.smp.forward_signal_enabled'

It seems this was an "emergency" knob to revert a newly introduced
behavior. Overall, we want better system-wide signal receive latency,
and it doesn't seem that some contrary policy was ever needed (and if
that comes up, it should rather be implemented, e.g., per-process).

Suggested by: kib
Reviewed by: kib, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42315