x86: Remove x86 specific GD flags as they are not referenced at all

This patch removes the x86 architecture specific GD flags
(GD_FLG_COLD_BOOT & GD_FLG_WARM_BOOT), as they are not used. Only
GD_FLG_COLD_BOOT is referenced in coreboot.c but assigned in start16.S.
But the coreboot target does not use start16.S at all and boots directly
from the 32-bit start code.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>