Remove definitions of MutationEvent and friends

Please see this WHATWG/dom issue for more details:

whatwg/dom#305 (comment)

Mutation events have now been disabled for most all users of
the Chrome browser. While there are still a few sites opted
back in to the events via origin trial tokens, those are time
limited and we've committed to end-date them as soon as possible.
However, it feels appropriate to remove the definition of these
deprecated events from the standard at this point.