Revert "Merge #10126: Compensate for memory peak at flush time"

Summary:
This reverts commit 7bd410c3c23067fcef2c6671676c104fbb8e974a.
Reverts D2999

After backporting PR10126 and then looking into PR10133 (https://github.com/bitcoin/bitcoin/pull/10133/files) it's clear that we've already backported up to this point and that re-implementing PR10126 is a step backwards.

Test Plan: make

Reviewers: deadalnix, Fabien, markblundeberg, #bitcoin_abc

Reviewed By: markblundeberg, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3007