chore: remove duplicate method

Author: Nicolò Maria Mezzopera

src/mixins/LayerGroup.js

@@ -12,9 +12,6 @@ export default {
   mounted () {
     this.layerGroupOptions = {...this.layerOptions};
   },
-  beforeDestroy () {
-    this.parentContainer.removeLayer(this);
-  },
   methods: {
     addLayer (layer, alreadyAdded) {
       if (!alreadyAdded) {