Release root components after they are destroyed

[mitchlloyd]

The first commit removes some guards around assertions that used to fail in Glimmer. They seem to be working now 🎉

The second commit removes destroyed root components from renderer._roots. This was the behavior before this commit.

I'm submitting this PR because retaining the destroyed components is causing trouble for me when attempting to create a new feature for ember-islands. I'd like to destroy a component and have it released so that I don't leak memory. The issue manifests in tests:

1. Use component.appendTo() in an acceptance test.
2. Call component.destroy().
3. On test teardown (application.destroy()) I get an error Error: Failed to execute 'removeChild' on 'Node': The node to be removed is not a child of this node.

Glimmer throws this error because Ember attempts to clean up all the _roots from _clearAllRoots. However the component in _roots has already been destroyed.

[rwjblue]

Sorry for letting this linger @mitchlloyd!

[mitchlloyd]

Thanks for reviewing!