validation, refactoring: remove ChainstateManager::Reset()

Summary:
remove ChainstateManager::Reset(), as it is currently unused (can be reintroduced in the test utilities if needed for unit testing)

Co-authored-by: Vasil Dimov <vd@FreeBSD.org>
Co-authored-by: laanwj <126646+laanwj@users.noreply.github.com>

This concludes backport of core#24299
https://github.com/bitcoin/bitcoin/pull/24299/commits/ae9ceed3e23288b163b7d7b1840b06b8d332f4ce

Depends on D12988

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12989