assert: apply minor refactoring

* Remove comment referring to the CommonJS Unit Testing 1.0 spec. This
  module is no longer intended to comply with that spec.
* Remove puzzling "THIS IS NOT TESTED NOR LIKELY TO WORK OUTSIDE V8!"
  comment. No doubt, it made sense at one time.
* Favor `===` over `==` in two places.

PR-URL: https://github.com/nodejs/node/pull/11511


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>