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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 5a8413e779a2258b02f92bad69a03d283c4bf2a6)