Merge #11889: Drop extra script variable in ProduceSignature

Summary:
9360f5032 Drop extra script variable in ProduceSignature (Russell Yanofsky)

Pull request description:

Was slightly confusing.

Tree-SHA512: 1d18f92c133772ffc8eb71826c8d778988839a14bcefc50f9c591111b0a5f81ebc12bca0f1ab25d5fdd02d3d50c2325c04cbfcbdcd18a7b80ca112d049c2327d

Backport of Core PR11889
https://github.com/bitcoin/bitcoin/pull/11889/

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3790