Remove dead code in core/events/.

This CL removes the following dead code:
- ClipboardEvent::create()
- CompositionEvent::CompositionEvent(const AtomicString&, PassRefPtr<AbstractView>, const String&)
- CompositionEvent::create(const AtomicString&, PassRefPtr<AbstractView>, const String&)
- Event::clipboardData
- Event::clipboard
- EventTarget::isFiringEventListeners
- GestureEvent::create()
- GestureEvent::initGestureEvent
- findKeyboardEvent in KeybaordEvent.{cpp,h}
- MouseEvent::create(const AtomicString&, bool, bool, PassRefPtr<AbstractView>,
                     int, int, int, int, int, int, int, bool, bool, bool, bool,
		     unsigned short, PassRefPtr<EventTarget>)
- MouseEvent::clipboard
- MouseRelatedEvent::pageLocation
- NodeEventContext::currentTarget
- NodeEventContext::treeScopeEventContext
- OverflowEvent::initOverflowEvent
- TextEvent::isBackTab

BUG=none
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/151123002

git-svn-id: svn://svn.chromium.org/blink/trunk@166274 bbb929c8-8fbe-4397-9dbb-9b2b20218538