Remove unnecessary variable `_hasPrimaryFocus` (#129066)

`_hasPrimaryFocus` variable and its related code is no longer needed after using `InkWell` for `DropdownButton` at https://github.com/flutter/flutter/pull/95906

packages/flutter/lib/src/material/dropdown.dart

@@ -1221,7 +1221,6 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
   Orientation? _lastOrientation;
   FocusNode? _internalNode;
   FocusNode? get focusNode => widget.focusNode ?? _internalNode;
-  bool _hasPrimaryFocus = false;
   late Map<Type, Action<Intent>> _actionMap;
 
   // Only used if needed to create _internalNode.
@@ -1244,14 +1243,12 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
         onInvoke: (ButtonActivateIntent intent) => _handleTap(),
       ),
     };
-    focusNode!.addListener(_handleFocusChanged);
   }
 
   @override
   void dispose() {
     WidgetsBinding.instance.removeObserver(this);
     _removeDropdownRoute();
-    focusNode!.removeListener(_handleFocusChanged);
     _internalNode?.dispose();
     super.dispose();
   }
@@ -1262,25 +1259,11 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
     _lastOrientation = null;
   }
 
-  void _handleFocusChanged() {
-    if (_hasPrimaryFocus != focusNode!.hasPrimaryFocus) {
-      setState(() {
-        _hasPrimaryFocus = focusNode!.hasPrimaryFocus;
-      });
-    }
-  }
-
-
   @override
   void didUpdateWidget(DropdownButton<T> oldWidget) {
     super.didUpdateWidget(oldWidget);
-    if (widget.focusNode != oldWidget.focusNode) {
-      oldWidget.focusNode?.removeListener(_handleFocusChanged);
-      if (widget.focusNode == null) {
-        _internalNode ??= _createFocusNode();
-      }
-      _hasPrimaryFocus = focusNode!.hasPrimaryFocus;
-      focusNode!.addListener(_handleFocusChanged);
+    if (widget.focusNode == null) {
+      _internalNode ??= _createFocusNode();
     }
     _updateSelectedIndex();
   }