delete MouseShadowElements of elements that where removed

[todo]

goldst.github.io/js/mouseShadow/MouseShadowController.js

Lines 11 to 16 in 4a86c4b

	* @todo delete MouseShadowElements of elements that where removed
	*/
	export default class MouseShadowController extends EC {
	/**
	* @param {object} baseElement - The common ancestor DOM element of
	* all shadowed elements. Attention: Depending on how many

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This issue was generated by todo based on a todo comment in 4a86c4b when #36 was merged. cc @goldst.