edison: Remove empty config header

Now that we support having CONFIG_SYS_CONFIG_NAME be unset to indicate a
lack of board.h file, unset this on the edison platform and remove
the otherwise empty file.

Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Tom Rini <trini@konsulko.com>

include/configs/edison.h

@@ -1,4 +0,0 @@
-/* SPDX-License-Identifier: GPL-2.0+ */
-/*
- * Copyright (c) 2017 Intel Corp.
- */