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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 1544f5add8c72c28e939a1557e3e319c6cfe5008)