[refactor] make InvalidChainFound part of CChainState's implementation

Summary:
This is only called by other methods of CChainState so make it private.

Depends on D7440

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7573