[electrum] remove ecdsa as a direct dependency

Summary:
This completes the work on removing the ecdsa dependency

It might still be needed indirectly as a dependency of a dependency (btchip-python, pystaochip.... require it).
Depends on D16705

Test Plan:
with and without the python ecdsa lib installed :

python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16707