refactor(chain)!: remove AnchorImpl wrapper for Anchor implementors
AnchorImpl was a wrapper created to allow the implementation of foreign
traits, like From/ToJson from serde_json for external unknown structs
implementing the Anchor trait.
As the Anchor generic in the rusqlite implementation for anchored
ChangeSets was the only place where this AnchorImpl was used and it has
been fixed to the anchor ConfirmationBlockTime, there is no more reason
to keep this wrapper around.