[Cashtab] Use ecash-script lib for op return tx parsing

Summary:
Improve handling of OP_RETURN parsing in Cashtab tx history by using the available getStackArray method from ecash-script.

Delete the now-unused parseOpReturn method and its associated tests and mocks.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15295