…ctive()

a0efe52 Fix outdated comments referring to ::ChainActive() (Samuel Dobson)

Pull request description:

  After bitcoin#21866 there are a few outdated comments referring to `::ChainActive()`, which should instead refer to `ChainstateManager::ActiveChain()`.

ACKs for top commit:
  jamesob:
    ACK bitcoin@a0efe52

Tree-SHA512: 80da19c105ed29ac247e6df4c8e916c3bf3f37230b63f07302114eef9c115add673e9649f0bbe237295be0c6da7b1030b5b93e14daf6768f17ce5de7cf2c9ff2

…lizeBlock

Summary:
This removes the last occurences of `::Chain(State)Active` in the code after D11704, D11742 and D11744 are landed. There are some remaining occurences in comments, which are removed in a later backport ([[bitcoin/bitcoin#23258 | core#23258]])

I checked in the codebase that these methods are only called on the active chainstate. It does not make much sense to finalize blocks in a chain base on the state of some other chain anyway.

Test Plan:
Before running the tests, add the following line in both modified methods:

`assert(std::addressof(m_chain) == std::addressof(::ChainActive()));`

`ninja all check-all`

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11748

Summary:
Remove also a comment about g_chainstate.
The additional `::ChainActive` comment removed in  validation.cpp  was missed in  a backport in D1972.

This is a backport of [[bitcoin/bitcoin#23258 | core#23258]]

Test Plan:
```
grep -r "::Chain\w*A" ../src/
grep -r "g_chainman" ../src/
```

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11755