[electrum] move-only: split bip32 from bitcoin.py

Summary:
Backport of https://github.com/spesmilo/electrum/commit/a88a2dea8255532abdd20002c866b443237a509c

Also remove unused function xpub_from_pubkey

Depends on D16278

Test Plan:
python test_runner.py

run the application from source, make sure the basic features are working

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16279