arm/SYS.h: remove unused CURBRK macro
The last use was removed in 2018 as part of a reimplementation of brk()
and sbrk() in commit 9d9fd255d646b6c389fa347cb633c665c4485aa4.
Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D43060