MeshBVHVisualizer: child root visualizations are not removed

[gkjohnson]

The roots are removed from the list but not the parent:

three-mesh-bvh/src/objects/MeshBVHVisualizer.js

Lines 260 to 264 in a6e9633

	while ( this._roots.length > totalRoots ) {
	this._roots.pop();
	}