remove unused __brk function/source file

commit e3bc22f1eff87b8f029a6ab31f1a269d69e4b053 removed all references
to __brk.

src/malloc/__brk.c

@@ -1,7 +0,0 @@
-#include <stdint.h>
-#include "syscall.h"
-
-uintptr_t __brk(uintptr_t newbrk)
-{
-	return __syscall(SYS_brk, newbrk);
-}