[ecash-herald] Do not use polling for the telegram bot
Summary: The bot only sends msgs, it does not need to monitor any msgs for pending actions. "polling" means periodically hitting the telegram API to check for new msgs. We should not do this as we do not need this feature in herald (unused).
Test Plan: npm test
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D17724