xmmap(): drop "Out of memory?"

We show that message with die_errno(), but the OS is ought to know
why mmap(2) failed much better than we do.  There is no reason for
us to say "Out of memory?" here.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -728,7 +728,7 @@ void *xmmap(void *start, size_t length,
 {
 	void *ret = xmmap_gently(start, length, prot, flags, fd, offset);
 	if (ret == MAP_FAILED)
-		die_errno("Out of memory? mmap failed");
+		die_errno("mmap failed");
 	return ret;
 }