target/arm: Drop ARM_FEATURE_XSCALE handling

We have now removed all the CPU types which had the Intel XScale
extensions indicated via ARM_FEATURE_XSCALE, so this feature bit
is never set. Remove all the code that can only be reached when
using this flag.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20250828140422.3271703-5-peter.maydell@linaro.org