ARM: vexpress_defconfig: Cleanup from non-existing options

ARCH_VEXPRESS_CA9X4 is gone since commit 81cc3f86 ("ARM: vexpress:
Remove non-DT code").

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>