feat(memory-leaks): remove elements from the window root

[BobrImperator]

• Adds removing elements explicitly from registries when unobserve is called.
• Adds removing elements from the default root (window)

In our testing this fixed memory problems related to modifiers that rely on this library.
e.g. ember-in-viewport and ember-scroll-modifiers
When benchmarking memory consumption was lowered from ~210MB to ~45MB.