HV: remove 'warm_reboot()'function and other minor cleanup

- 'reboot.c' defines 'warm_reboot()' only, and this
    function is deprecated.
  - remove the eXecution attribute of 'mttr.h'

Signed-off-by: Yonghua Huang <yonghua.huang@intel.com>
Acked-by: Eddie Dong <eddie.dong@intel.com>

hypervisor/arch/x86/debug/reboot.c

@@ -1,12 +0,0 @@
-/*
- * Copyright (C) <2018> Intel Corporation
- * SPDX-License-Identifier: BSD-3-Clause
- */
-#include <hypervisor.h>
-#include <reboot.h>
-
-int warm_reboot(void)
-{
-	pio_write8(0x6, 0xcf9);
-	return 0;
-}