doc: Remove irrelevant link to GitHub

Summary:
The doc nicely explains why the directory exists and it is irrelevant when it was introduced. Even if it was relevant, it could be trivially found out via git log ./src/node/ | tail without visiting GitHub

This concludes backport of core#21575 [6/6]
https://github.com/bitcoin/bitcoin/pull/21575/commits/fadcd3f78e1dd1acd7a774f8fad68dc471ff9e1f

Test Plan: NA

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11352