Drop special handling of SIGSYS.

Drop special handling of SIGSYS.

Tolerance for up to 25 SIGSYS deliveries was added to init(8) soon after
the addition of sysctl(2) at CSRG, presumably to ease the transition to
the new ABI.

After 25 years of work the ABI transition is finally complete and we can
now safely remove this splint.

In general, we now have better practices and methods for helping userspace
across kernel ABI breaks.

ok deraadt@