Remove unused variable

Summary: This is a partial backport of Core PR15759 : https://github.com/bitcoin/bitcoin/pull/15759/commits/26a93bce29fd813e1402b013f402869c25b656d1

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D6399