remove unused __brk function/source file

commit e3bc22f1 removed all references
to __brk.

src/malloc/__brk.c

-#include <stdint.h>
-#include "syscall.h"
-
-uintptr_t __brk(uintptr_t newbrk)
-{
-	return __syscall(SYS_brk, newbrk);
-}