target-arm: Remove out of date FIXME regarding saturating arithmetic

Remove an out of date FIXME regarding the saturating arithmetic helpers:
we now do pass a pointer to CPUARMState to these helpers, and since
the AREG0 changes went in there is no difference between helper.c
and op_helper.c and therefore no point in moving the functions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>