[electrum] regtest: refactor poll_for_answer

Summary:
Make the function easier to understand and remove a few indentation levels by removing the unnecessary retry flag and reorganizing the code a bit.
There is no change in behavior.

Depends on D14838

Test Plan: pytest electrumabc/tests/regtest

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14839