[seeder] Remove variable that is never read

Summary:
This is a partial backport of bchn#1820
https://gitlab.com/bitcoin-cash-node/bitcoin-cash-node/-/merge_requests/1820/diffs?commit_id=76c7b65a1e3016340aa845ac83f341414326fb59

Depends on D16904

Test Plan: grep -r nDirty src/seeder

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16905