Retire TimedItem events and obsolete names

Retire long-deprecated features
  TimedItem events (start, iteration, end, cancel)
  window.Timeline
  TimedItem.specified

Retiring TimedItem events involves retiring
  TimedItem.addEventListener
  TimedItem.removeEventListener
  TimedItem._generateChildLegacyEventsForRange
  TimedItem._generateLegacyEvents
  TimedItem._hasLegacyEventHandlers
  AnimationPlayer._checkForLegacyHandlers()
  AnimationPlayer._legacyHandlerAdded()
  AnimationPlayer._needsLegacyHandlerPass
and retiring the tests
  impl-test-player-sees-handlers.html
  test-timeditem-addeventlistener.html
  test-timeditem-events.html