Bug 1031094 - Uninstalling a collection web result unpins it from collections. r=kgrandon

Ok, not sure if you saw, but here is what the old homescreen was doing: https://github.com/mozilla-b2g/gaia/blob/master/apps/homescreen/test/marionette/lib/bookmark_editor.js#L6

I haven't had a chance to look into the details of the selectors, but I suppose this would be fine for now.