Resolve "Unused function ivas_param_ism_dec()"

• Related issues: #1022 (closed)
• Requested reviewers: @bayers, @eichenseer

Reason why this change is needed

• The function ivas_param_ism_dec() is unused.

Description of the change

• The function ivas_param_ism_dec() is removed.

Affected operating points

• All is BE.