[Cashtab] [local opreturn encode p1/2] Deprecate BCH.script.encode

Summary:
T2730

Deprecate BCH.Script.encode

Because this function is already involved in several specific unit tests, and this diff ensures that they still pass, there was no reason to add an intermediate stack comparing the two functions in the dev console.

Test Plan:
npm test
npm start and send some encrypted and unencrypted msgs

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12629