doc: remove GitHub mark

Judging from https://github.com/logos, we are misusing the GitHub mark.
That page indicates to not change the color (we change it from black to
green). Less clear, it says  "Use the Mark in social buttons to link to
your GitHub profile or project" which isn't exactly what we're doing but
also isn't not what we're doing?

This might be an indication of my eyesight getting worse as I get older,
but I think the size that we display the mark at by default makes it not
entirely recognizable as the GitHub mark. Lastly, there's the
philosophical issue of whether we should display a commercial entity's
mark on our web pages in this particular instance. (For me, the answer
is "maybe".)

All told, I think we can remove it without damaging usability on the
website, so let's do it.

PR-URL: https://github.com/nodejs/node/pull/39251


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>