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/offcanvas.js

@@ -162,9 +162,6 @@ class Offcanvas extends BaseComponent {
     this._backdrop.dispose()
     super.dispose()
     EventHandler.off(document, EVENT_FOCUSIN)
-
-    this._config = null
-    this._backdrop = null
   }
 
   // Private