Remove event deprecations from core

This removes all of them but one major one: https://github.com/atom/atom/blob/master/src/pane-view.coffee#L159-L160

I wasnt sure how we wanted to handle this, as pane items can be anything. Do we still want to support the path-changed event on random pane items? Do we want to enforce an `onDidChangeTitle` method? This is one case where string events are really nice.
I don't mind the caution, but I think if they implement an `::on` method the thing to do is encourage them to implement a named method rather than worry about the returned disposable.