Clean up MozL10n API use in Cost Control #1057795

This is relating bug 1057795
I've seen how `_` is removed from global definitions and that's ok but if we are removing **all** appearances of this function then we must to remove its declaration in [`js/utils/toolkit.js:5`](https://github.com/mozilla-b2g/gaia/blob/master/apps/costcontrol/js/utils/toolkit.js#L5) and in [`js/settings/settings.js:23`](https://github.com/mozilla-b2g/gaia/blob/master/apps/costcontrol/js/settings/settings.js#L23) and notice this probably breaks a lot of test so you need to rewrite them.