assumeutxo: remove snapshot during -reindex{-chainstate}

Summary:
Removing a snapshot chainstate from disk (and memory) is consistent with
existing reindex operations.

This is a partial backport of core#27596
https://github.com/bitcoin/bitcoin/pull/27596/commits/c711ca186f8d8a28810be0beedcb615ddcf93163

Depends on D17632

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, roqqit

Reviewed By: #bitcoin_abc, roqqit

Differential Revision: https://reviews.bitcoinabc.org/D17633