Remove unused ISAR functions

• Related issues: sa4/audio/ivas-basop#2371 (closed)
• Requested reviewers: @tamarapu

Reason why this change is needed

Some ISAR-related BASOP functions are obsolete and unused, e.g. PerceptualMode() or PerceptualModelStereo(). They are replaced by _fx variants. They can be removed.

Description of the change

Remove obsolete and unused functions

Affected operating points

All BE.

Closes #2371