Move ChainImpl from interfaces/chain.cpp to node/interfaces.cpp

Summary:

No changes to ChainImpl or any related classes (review with git diff --color-moved=dimmed_zebra)

Also remove unused includes.

This is a partial backport of core#20494
https://github.com/bitcoin/bitcoin/pull/20494/commits/2a26771d8161d30be1853a35acfee588cce03634

Depends on D12746

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12747