[non-BE] Resolve "Discrete OSBA Stereo output missing scaling factor of 0.5"

• Related issues: #1262 (closed)
• Requested reviewers: @weckbecker

Reason why this change is needed

• See issue description

Description of the change

• Missing scaling factor was added.
• Redundant code in ivas_osba_stereo_add_channels() was removed.

Affected operating points

• Discrete OSBA Stereo output

Related to #1262 (closed)