[Cashtab] Remove unnecessary wrapper function

Summary:
T3154

This was needed once upon a time but its functionality is now included in the utxo-lib library.

Test Plan:
npm test
cd cashtab/ && grep -r encodeOpReturnScript src/ and no output

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14314