[web-animations-1] Add AnimationEffect.remove()?

[birtles]

See issue at: https://drafts.csswg.org/web-animations-1/#animationeffectreadonly

Specifically (since that section might soon disappear):

The remove() method can be used to remove an effect from either its parent group or animation. Should we keep it in level 1 and define it simply as removing the animation from its animation?