Obsolete function ivas_spar_dec_upmixer()

The function ivas_spar_dec_upmixer() is defined but never used - see also the code coverage at https://ivas-codec-pc.3gpp.org/ivas-codec/coverage/lib_dec/ivas_spar_decoder.c.gcov.html.

It is proposed to clean it out and report this on BASOP issue tracker.