[chronik-docs] Fix a wrong link in the websocket documentation

Summary:
There is a remaining link that refers to an _InNode class (deprecated since NNG is no longer supported).
This also removes all the remaining _InNode references in the repo (only comments apart from this link).

Test Plan: Grepping for _InNode returns a single result which is a changelog

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17166