coins: call ClearFlags in CCoinsCacheEntry destructor

Summary:
No behavior change. Prepares for flags adding CCoinsCacheEntrys
to a linked list which need to be removed on destruction.

This is a partial backport of core#28280
https://github.com/bitcoin/bitcoin/pull/28280/commits/58b7ed156d5993b69375bb455b03bd8b17f64fa4

Depends on D18615

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18616