[css-transitions] changed transitionend event not to be cancellable

In https://dvcs.w3.org/hg/csswg/rev/c68eecb7c288 I changed the
transitionend event to not be Cancelable.  This fixes
https://www.w3.org/Bugs/Public/show_bug.cgi?id=21101 where Anne
points out that it doesn't make sense for it to be cancelable since
it has no default action, so there's no meaning to cancelling it.

It also makes it consistent with the animation events.

-David

-- 
𝄞   L. David Baron                         http://dbaron.org/   𝄂
𝄢   Mozilla                          https://www.mozilla.org/   𝄂
             Before I built a wall I'd ask to know
             What I was walling in or walling out,
             And to whom I was like to give offense.
               - Robert Frost, Mending Wall (1914)