Merge #12693: Remove unused variable in SortForBlock

Summary:
bb079a0e2c Remove unused variable in SortForBlock (Drew Rasmussen)

Pull request description:

Although txiter is passed to BlockAssembler::SortForBlock, it is never used. Other than BlockAssembler::addPackageTxs, no other method ever makes a call to SortForBlock, thus making this change harmless.

Tree-SHA512: c7df948c5f75f7371844200e0227a26476437f300148d29020e01041b382f5bda31d9c520c9c5425aee88ce8f4a52cd0e594985d69ed8a081b878cda2e4de8c5

Backport of Core PR12693
https://github.com/bitcoin/bitcoin/pull/12693/

Test Plan:

make check

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

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3762