get rid of obsolete syscall header

neo is built on top of libc now

include/neo/_unistd.h

@@ -1,24 +0,0 @@
-/** See the end of this file for copyright and license terms. */
-
-#pragma once
-
-#include "neo.h"
-
-long _neo_syscall(long number, ...);
-
-isize _neo_sys_write(int fd, const void *buf, usize count);
-__attribute__((__noreturn__))
-void _neo_sys_exit(int status);
-
-/*
- * This file is part of libneo.
- * Copyright (c) 2021 Fefie <owo@fef.moe>.
- *
- * libneo is non-violent software: you may only use, redistribute,
- * and/or modify it under the terms of the CNPLv6+ as found in
- * the LICENSE file in the source code root directory or at
- * <https://git.pixie.town/thufie/CNPL>.
- *
- * libneo comes with ABSOLUTELY NO WARRANTY, to the extent
- * permitted by applicable law.  See the CNPLv6+ for details.
- */