[herald] Remove unused flag from parsedTx data

Summary:
T2972

Depends on D13715

isTokenTx is a useful variable as a private flag in parseTx. It isn't used outside of the function so is not needed in the parsedTx model.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13716