target-arm: Remove out of date FIXME regarding saturating arithmetic

Remove an out of date FIXME regarding the saturating arithmetic helpers: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: NPeter Maydell <peter.maydell@linaro.org>

target-arm/op_helper.c

@@ -93,8 +93,6 @@ void tlb_fill(CPUARMState *env, target_ulong addr, int is_write, int mmu_idx,
 }
 #endif
 
-/* FIXME: Pass an explicit pointer to QF to CPUARMState, and move saturating
-   instructions into helper.c  */
 uint32_t HELPER(add_setq)(CPUARMState *env, uint32_t a, uint32_t b)
 {
     uint32_t res = a + b;