BB 43795: Restore the URL classifier XPCOM components.

We are not really interested in removing them.
We're interested in them not calling Mozilla.
However, we have a central patch for RemoteSettings, so we do not need
to patch the single user.
Or, if we did, at least we should patch
UrlClassifierExceptionListService where it creates its RemoteSettings
instance.

BB 43795: Restore the URL classifier XPCOM components.

Remove this page as part of another commit.

This comit should go away automatically at the next rebase.

BB 43795: Restore the URL classifier XPCOM components.

Move the commit where we disable about:urlclassifier.

Make explicit that blanking REMOTE_SETTINGS_SERVER_URL is our change.