Remove unnecessary blockfile SetPos

Summary:

Nothing could have changed the position since we retrieved it a few statements earlier. This dates from commit https://github.com/bitcoin/bitcoin/commit/16d5194165c8c83492b95f431a664d98c40ff254 .

This is a backport of core#18984

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D10017