[Cashtab] [local opreturn encode p2/2] Simplify function

Summary:
T2730

Depends on D12629

Remove the ambiguous data variable and simply return the encoded value

Test Plan:
npm test
npm start and send an xec tx with an OP_RETURN msg

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12639