fs: Revert throw on invalid callbacks

This reverts 4cb5f3da

Based on community feedback I think we should consider reverting this
change. We should explore how this could be solved via linting rules.

Refs: https://github.com/nodejs/node/pull/12562
PR-URL: https://github.com/nodejs/node/pull/12976


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>