math/stp: remove nonsensical sbrk use

This code called sbrk(4000000) and beyond error checking ignored
the result meaning the 4MB of address space allocated could
never be used without another call to sbrk and reference to
INITIAL_MEMORY_PREALLOCATION_SIZE, but nothing in the repo does
either.

Submitted upstream as https://github.com/stp/stp/pull/474

Approved by: arrowd (maintainer)
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42978