Clear .root style class from a root node that is removed from a Scene/SubScene

When a `Node` is set as the root node of a `Scene` or `SubScene`, the "root" style class is automatically added to the node, but not cleared when the node is later removed from the scene.

This can lead to an incorrectly set "root" style class when the removed node is inserted in the scene graph below the root node, or to duplicate "root" style class entries when the removed node is again set as the root node of the scene.