[electrum] move Transaction.serialize_output() to TxOutput.serialize()

Summary:
Remove Transaction.serialize_output and inline Transaction.pay_script where it is needed.

Depends on D14501

Test Plan: python test_runner.py

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14502