[electrum] move ECKey.verify_message to ECPubkey.verify_message_for_address

Summary:
The ECKey class is about to be replaced. And we don't need a private key to verify a message anyway.

Depends on D16351

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16352