code coverage: OSBA

The code coverage revealed an obsolete code in ivas_enc() related to OSBA - see https://ivas-codec-pc.3gpp.org/ivas-codec/coverage/lib_enc/ivas_enc.c.gcov.html. Basically, ivas_sce_enc() and ivas_cpe_enc() are never employed in ISM_SBA_MODE_DISC mode.

Further, it would be nice to have common core-coder function calls regardless of the ism_mode at the encoder, similarly as it is done at the decoder.

Next, I propose to harmonize the code related to the writing of SBA signaling bits, i.e. remove the duplicated code and call the signaling at one common place inside of ivas_spar_enc().