Unused if-macro code in ivas_dirac_dec_binaural_functions_fx.c

Task

There is a bit of code in ivas_dirac_dec_binaural_functions_fx.c delimited with #if ( BINAURAL_CHANNELS != 2 ) that is pretty much unused. I confirmed with @multrus that this a leftover of optimization work. As that path should never be true, this can be removed in cleanup.