neon: disable asm quantizer

The assembly version of the quantizer has not been updated to match
the new exact quantizer introduced in commit e04e2935. That commit tried
to disable this code but missed the non-RTCD case.

Thanks to David Baker <david.baker at openmarket.com> for isolating the
issue and testing this fix.

Change-Id: I0e51492dc6f8e44d2c10b587427448bf94135c65