Remove risky compatability with old kernels

The badsys() handler for SIGSYS was added as a transtion aid for kernels
lacking sysctl() in 1993. It is unsafe and unsound so remove it rather
than running the risk of a privilege-dropping system call being silently
omitted.

This partially reverts SCCSID 6.12 (Berkeley) 03/03/93 "add code to
change the system security level".

Reviewed by: mckusick, imp, kevans
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D26289