Remove safebrowsing URLs

Every now and then, users find safebrowsing URLs by typing "google" in about:config, and are upset about it.

We disable safebrowsing (and in any case we don't even have an API key to use it), so it's more a perception matter (and at the end of the day it could be seen also as "defense in depth").

In any case, since we have seen this more than once, I think it's fair to just empty the URLs, even though not strictly needed from a technical point of view.