doc: remove tentativeness in pull-requests.md

There are some uses of "in general" that are unnecessary. (We don't need
to be tentative about the fact that tests should pass, for example.)

PR-URL: https://github.com/nodejs/node/pull/19123


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>