doc: remove pull-requests.md preamble

Refs: https://github.com/nodejs/TSC/issues/864#issuecomment-628646560

PR-URL: https://github.com/nodejs/node/pull/36960


Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>