[electrum] remove unused objects and methods in wallet

Summary:
Nothing in this codebase needs the old versions of the code, except for one callsite in main_window which is easy to update to use the new version.
Remove the technical debt.

Test Plan:
python electrum/test_runner.py

Test run the application

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14229