Refactor: move disposing properties into the base class (#33740)

Moves more functionality to `base-component`, transferring the responsibility of disposal to parent class.
Each component, dusting disposal, sets its protected properties to `null`. So the same can be done in one place for all children components .

js/src/dropdown.js

@@ -217,11 +217,8 @@ class Dropdown extends BaseComponent {
   }
 
   dispose() {
-    this._menu = null
-
     if (this._popper) {
       this._popper.destroy()
-      this._popper = null
     }
 
     super.dispose()