Fix memory leak by pruning event tree in unbind.
Fix memory leak by pruning event tree in unbind.

The unbind method only removes target events from the tree, leaving
empty nodes in it's path to accumulate. This fix collects the inner
nodes and iterates over them in reverse to check for and remove empty
nodes after an event is unbound.

Fixes adobe-webplatform/Snap.svg#369

adobe-webplatform/eve,adobe-webplatform/eve