setrlimit: Remove special handling for RLIMIT_STACK with a stack gap

This will not be required with a forthcoming reimplementation of ASLR
stack randomization. Moreover, this change was not sufficient to enable
the use of a stack size limit smaller than the stack gap itself.

PR: 260303
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33704