[electrum] make config parameter non-optional for instantiating a network

Summary:
This "feature" was only used in a script, so just instantiate configuration there and pass it to the network.

Depends on D18779

Test Plan:

python scripts/get_history.py ecash:qqlz0neyh0z8epd9zu6rmjankgvpnksltqljeg4apc

Run the application, check for no python errors on startup

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18780