[electrum] move-only: move the early exit conditions before the broadcast functions in broadcast_transaction

Summary:
No need to define broadcast_thread or and broadcast_done local functions if we are not going to use them because there is no network or the fee is too low.

Depends on D14847

Test Plan: send a transaction

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14848