Unused function ivas_param_mc_get_mono_stereo_mixing_matrices()

The function ivas_param_mc_get_mono_stereo_mixing_matrices() is unused and can be removed because TCs are already the DMX to mono or stereo. In this case, buffer_mode == TC_BUFFER_MODE_BUFFER and ivas_jbm_dec_tc_buffer_playout() is called as a decoder.

See https://ivas-codec-pc.3gpp.org/ivas-codec/coverage/lib_dec/ivas_mc_param_dec.c.gcov.html.

(This issue replicates #278 (closed) and #321 (closed)).