Revert "kern_exec: Add kern.stacktop sysctl."

The current ASLR stack gap feature will be removed, and with that the
need for the kern.stacktop sysctl is gone. All consumers have been
removed.

This reverts commit a97d697122da2bfb0baae5f0939d118d119dae33.

Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33704