MDCT Stereo mode force active

We noted that the define DEBUG_FORCE_MDCT_STEREO_MODE is enabled in options.h. Since it is a debugging option, this does not seem right. The current "Blame" attributes this to @svedberg, but tracing the changes back reveals that it was enabled in the initial baseline. It may have been a whitespace change that changed the Blame.

If my understanding is right, this has now been enabled during the selection testing. I don't know if it should remain active or if it should be switched off. I did not check the impact of this switch, but we noted that it is at least not bit exact.