[NFC] Remove unused headers from OpenBSD PAL.

src/pal/pal_openbsd.h

@@ -1,14 +1,8 @@
 #pragma once
 
 #if defined(__OpenBSD__) && !defined(_KERNEL)
-#  include "../ds/bits.h"
-#  include "../mem/allocconfig.h"
 #  include "pal_bsd.h"
 
-#  include <stdio.h>
-#  include <string.h>
-#  include <sys/mman.h>
-
 namespace snmalloc
 {
   /**