validation: Make PruneOneBlockFile() a member of ChainstateManager
Summary:
Partial backport (4/6) of core PR18698:
https://github.com/bitcoin/bitcoin/pull/18698/commits/fa24d4909864096934577abc26cfa9be47f634ba
Depends on D8562.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D8563