[electrum] start using tx.txinputs() in the wallet

Summary:
Convert all callsites from Transaction.inputs() to Transaction.txinputs(). The latter is less expensive as it does not require a dict building operation with hex conversions (especially when all we are interested in is len(tx.txinputs())

Depends on D14521

Test Plan:
python test_runner.py

double-click on a transaction in the history to exercise get_wallet_delta

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14520