Remove risky compatability with old kernels

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