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

sys/kern/subr_smp.c

@@ -113,12 +113,6 @@ SYSCTL_INT(_kern_smp, OID_AUTO, topology, CTLFLAG_RDTUN, &smp_topology, 0,
     "Topology override setting; 0 is default provided by hardware.");
 
 #ifdef SMP
-/* Enable forwarding of a signal to a process running on a different CPU */
-static int forward_signal_enabled = 1;
-SYSCTL_INT(_kern_smp, OID_AUTO, forward_signal_enabled, CTLFLAG_RW,
-	   &forward_signal_enabled, 0,
-	   "Forwarding of a signal to a process on a different CPU");
-
 /* Variables needed for SMP rendezvous. */
 static volatile int smp_rv_ncpus;
 static void (*volatile smp_rv_setup_func)(void *arg);
@@ -205,8 +199,6 @@ forward_signal(struct thread *td)
 
 	if (!smp_started || cold || KERNEL_PANICKED())
 		return;
-	if (!forward_signal_enabled)
-		return;
 
 	/* No need to IPI ourself. */
 	if (td == curthread)