Remove argument passed to the `@destroy` event

[filipsobol]

The @destroy event has one argument, which was meant to be the editor instance. However, because this instance is removed before the event is fired, the argument is always null.

ckeditor5-vue/src/ckeditor.ts

Lines 164 to 171 in d3f27a6

	if ( this.instance ) {
	this.instance.destroy();
	this.instance = null;
	}
	// Note: By the time the editor is destroyed (promise resolved, editor#destroy fired)
	// the Vue component will not be able to emit any longer. So emitting #destroy a bit earlier.
	this.$emit( 'destroy', this.instance );