[electrum] remove server.relay_fee API call

Summary: The relay_fee is unused, so no need to ask the server about it.

Test Plan: Run the Application for a few minutes, connect to another fulcrum server, wait a few minutes, make sure basic features still work

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16393