Disable redundant Kinto based blocklist update mechanism

Mozilla is in the transition to move to a new blocklist update system based on Kinto which is more fine-grained as the old one that used a big XML document, see: https://wiki.mozilla.org/Firefox/Kinto

It seems both systems are running in parallel right now but only the old one is used (see: notify() in nsBlocklistService.js) (I guess we are still in Phase 1 as outlined on the above wiki page). We should disable the one based on Kinto as there is no need to do additional requests and be affected by possible Kinto-related code issues.

Flipping services.blocklist.update_enabled to false should be enough I think:

    // If kinto update is enabled, do the kinto update
    if (gPref.getBoolPref(PREF_BLOCKLIST_UPDATE_ENABLED)) {
      const updater =
        Components.utils.import("resource://services-common/blocklist-updater.js",
                                {});
      updater.checkVersions().catch(() => {
        // Before we enable this in release, we want to collect telemetry on
        // failed kinto updates - see bug 1254099
      });
    }