Adding -stereo_dmx_evs should remove -stereo for IVAS and use EVS decoding

Using the cod: opts: key to add -stereo_dmx_evs adds it to the commandline, but since the input format is stereo, both arguments are added:

./IVAS_cod -stereo_dmx_evs -stereo ...

This causes the encoder to run in Stereo mode. If this special argument is added, then -stereo should be checked for and removed.

Further, the EVS decoder should be used to decode this bitstream.