[ecash-herald] Remove deprecated function

Summary:
T2972

With last couple of refactors to implement ecash-script lib, this function is no longer used

Test Plan:
npm test

grep -r hexOpReturnToUtf8 src/ and no output
grep - hexOpReturnToUtf8 test/ and no output

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D14033