Avoid scheduling additional runloop timers in willDestroyElement
Avoid scheduling additional runloop timers in willDestroyElement

Ember does not set the isDestroying flag when willDestroyElement is called,
meaning the guards on hide() are not triggered, resulting in timers being
scheduled. Luckily, we don't need to call the full hide(), we just need to
cancel some timers. We call dispose() on the tooltip.js instance, which
already handles [hiding the tooltip](https://github.com/FezVrasta/popper.js/blob/b98d9134eb78b97e8bc1b736401a960e9e3cb9f6/packages/tooltip/src/index.js\#L278)

This fixes an issue where the timers set during tooltip teardown could
bleed into other acceptance tests and wreck havoc with runloop waiting
behavior.

sir-dunxalot/ember-tooltips,sir-dunxalot/ember-tooltips