Unused function ivas_param_ism_dec()

The function ivas_param_ism_dec() is unused and can be removed because it was replaced by ivas_param_ism_dec_digest_tc().

Similarly the function ivas_param_ism_rendering() is unused and can be removed because it was replaced by ivas_param_ism_render_slot().

In addition, in the function ivas_param_ism_params_to_masa_param_mapping() there is a condition if ( st_ivas->nchan_ism > 1 ) ... else ... which has no sense because in ParamISM, nchan_ism is always 3 or 4.

See https://ivas-codec-pc.3gpp.org/ivas-codec/coverage/lib_dec/ivas_ism_param_dec.c.gcov.html for coverage test results.