2.0: Remove DOMContentLoaded helper

Is <code>DOMContentLoaded</code> helper really needed? In 2.0, the only reason for its presence is to clean-up <code>DOMContentLoaded</code> listener, whereas <code>load</code> event is not cleared, so why one is removed and the other doesn't?

I think jQuery should clear either both events or none of them, in this pull i propose the second one.

/cc @rwldrn