Tiny remove unnecessary method in text_selection.dart (#127480)

I come across this when migrating my codebase. There is indeed a linter rule for it: unnecessary_overrides (see screenshot below for an example). I can help enabling the rule (as well as other rules, and/or dart formatter, etc) for Flutter codebase if needed.

![image](https://github.com/flutter/flutter/assets/5236035/3fe8fa32-cde2-45a9-901a-a383e8457fa1)

packages/flutter/lib/src/widgets/text_selection.dart

@@ -3080,11 +3080,6 @@ class _TextSelectionGestureDetectorState extends State<TextSelectionGestureDetec
     }
   }
 
-  @override
-  void dispose() {
-    super.dispose();
-  }
-
   // The down handler is force-run on success of a single tap and optimistically
   // run before a long press success.
   void _handleTapDown(TapDragDownDetails details) {