arm: vexpress64: Remove unneeded CONFIG_ check

CONFIG_SEMIHOSTING is selected for the VFP target by the means of
Kconfig already, there is no need to check this in the header file.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>