doc: remove squash guideline from onboarding doc

Although I agree with the guideline people should generally not squash
commits in a pull request until the end (in other words, until it's time
to land the PR), it is clear from comments and actions in the issue
tracker that many do not share that view. This is fine by me, but I do
think that we should our documentation should reflect our practices
rather than being an aspirational statement.

If we *do* wish to preserve this recommendation, it probably belongs in
another document anyway as this is not a recommendation for
Collaborators only but for anyone opening a pull request.

PR-URL: https://github.com/nodejs/node/pull/20413


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>