[backport#16194] refactoring: make pindexBestInvalid internal to validation.cpp

Summary:
There's no need to have this member live on CChainState since it's only used
in validation.cpp.

https://github.com/bitcoin/bitcoin/pull/16194/commits/55d525ab9004631d30dcc60a1ec5d9cd6c6afe56

Depends on D6967

Partial backport of Core PR16194

Test Plan:

ninja check && ninja check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6968