[electrum] remove parse_scriptSig function

Summary:
Use TxInput.parse_scriptsig instead.

Depends on D14458

Test Plan:
python test_runner.py

Send transactions in the application (regular wallet and multisig)

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14459