doc: Remove irrelevant link to GitHub

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

Author: MarcoFalke

src/node/README.md

@@ -15,8 +15,7 @@ As a rule of thumb, code in one of the [`src/node/`](./),
 calling code in the other directories directly, and only invoke it indirectly
 through the more limited [`src/interfaces/`](../interfaces/) classes.
 
-The [`src/node/`](./) directory is a new directory introduced in
-[#14978](https://github.com/bitcoin/bitcoin/pull/14978) and at the moment is
+This directory is at the moment
 sparsely populated. Eventually more substantial files like
 [`src/validation.cpp`](../validation.cpp) and
 [`src/txmempool.cpp`](../txmempool.cpp) might be moved there.