Since we dropped darwin-user support many years ago, the code in
user-exec to support hosts which define __APPLE__ is unused; delete it.

Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>