x86: coreboot: delete unused coreboot/config.mk

HOSTCFLAGS_autoconf.mk.dep was added by commit 422322f
but it has never been used.

Cc: Vadim Bendebury <vbendeb@chromium.org>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

board/chromebook-x86/coreboot/config.mk

@@ -1,7 +0,0 @@
-#
-# Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
-#
-# SPDX-License-Identifier:	GPL-2.0	BSD-3-Clause
-#
-
-HOSTCFLAGS_autoconf.mk.dep = -Wno-variadic-macros