x86: Delete redundant compiler flags

-Wstrict-prototypes, -ffreestanding, -fno-stack-protector
are defined at the top Makefile for all architectures.

Do not define them twice for x86.

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