[electrum] remove more dynamic fee related dead code

Summary:
Most of it was deleted in https://github.com/Bitcoin-ABC/ElectrumABC/pull/45
This was left over.

Removing this has the side benefit of getting rid of a translation hack in util.py

Test Plan: Run the application, play with the fee slider, go to preferences and select custom fee rate and type something.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18319