[improvement] Checkbox: use relation mixin

packages/mixins/checkbox.js

@@ -2,10 +2,10 @@
  * Common part of Checkbox & Radio
  */
 import Icon from '../icon';
-import { FindParentMixin } from './find-parent';
+import { ChildrenMixin } from './relation';
 
 export const CheckboxMixin = (parent, bem) => ({
-  mixins: [FindParentMixin],
+  mixins: [ChildrenMixin(parent)],
 
   props: {
     name: null,
@@ -20,10 +20,6 @@ export const CheckboxMixin = (parent, bem) => ({
     }
   },
 
-  created() {
-    this.findParent(parent);
-  },
-
   computed: {
     isDisabled() {
       return (this.parent && this.parent.disabled) || this.disabled;