Revert: Fix DropdownMenu rendered behind AppBar (#159907)

Reverts https://github.com/flutter/flutter/pull/155539.

The PR was found to have caused regression
https://github.com/flutter/flutter/issues/159906.

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

diff --git a/packages/flutter/lib/src/material/menu_anchor.dart b/packages/flutter/lib/src/material/menu_anchor.dart
index 4248ae2..568b812 100644
--- a/packages/flutter/lib/src/material/menu_anchor.dart
+++ b/packages/flutter/lib/src/material/menu_anchor.dart
@@ -406,7 +406,7 @@
       );
     }
 
-    Widget child = OverlayPortal.targetsRootOverlay(
+    Widget child = OverlayPortal(
       controller: _overlayController,
       overlayChildBuilder: (BuildContext context) {
         return _Submenu(

packages/flutter/test/material/menu_anchor_test.dart

diff --git a/packages/flutter/test/material/menu_anchor_test.dart b/packages/flutter/test/material/menu_anchor_test.dart
index 4216f2a..83e312a 100644
--- a/packages/flutter/test/material/menu_anchor_test.dart
+++ b/packages/flutter/test/material/menu_anchor_test.dart
@@ -4688,83 +4688,6 @@
     expect(iconStyle(tester, Icons.add).color, disabledIconColor);
   });
 
-  // Regression test for https://github.com/flutter/flutter/issues/155034.
-  testWidgets('Content is shown in the root overlay', (WidgetTester tester) async {
-    final MenuController controller = MenuController();
-    final UniqueKey overlayKey = UniqueKey();
-    final UniqueKey menuItemKey = UniqueKey();
-
-    List<RenderObject> ancestorRenderTheaters(RenderObject child) {
-      final List<RenderObject> results = <RenderObject>[];
-      RenderObject? node = child;
-      while (node != null) {
-        if (node.runtimeType.toString() == '_RenderTheater') {
-          results.add(node);
-        }
-        final RenderObject? parent = node.parent;
-        node = parent is RenderObject? parent : null;
-      }
-      return results;
-    }
-
-    late final OverlayEntry overlayEntry;
-    addTearDown((){
-      overlayEntry.remove();
-      overlayEntry.dispose();
-    });
-
-    Widget boilerplate() {
-      return MaterialApp(
-        home: Overlay(
-          key: overlayKey,
-          initialEntries: <OverlayEntry>[
-            overlayEntry = OverlayEntry(
-              builder: (BuildContext context) {
-                return Scaffold(
-                  body: Center(
-                    child: MenuAnchor(
-                      controller: controller,
-                      menuChildren: <Widget>[
-                        MenuItemButton(
-                          key: menuItemKey,
-                          onPressed: () {},
-                          child: const Text('Item 1'),
-                        ),
-                      ],
-                    ),
-                  ),
-                );
-              }
-            ),
-          ],
-        ),
-      );
-    }
-
-    await tester.pumpWidget(boilerplate());
-    expect(find.byKey(menuItemKey), findsNothing);
-
-    // Open the menu.
-    controller.open();
-    await tester.pump();
-    expect(find.byKey(menuItemKey), findsOne);
-
-    // Expect two overlays: the root overlay created by MaterialApp and the
-    // overlay created by the boilerplate code.
-    expect(find.byType(Overlay), findsNWidgets(2));
-
-    final Iterable<Overlay> overlays = tester.widgetList<Overlay>(find.byType(Overlay));
-    final Overlay nonRootOverlay = tester.widget(find.byKey(overlayKey));
-    final Overlay rootOverlay = overlays.firstWhere((Overlay overlay) => overlay != nonRootOverlay);
-
-    // Check that the ancestor _RenderTheater for the menu item is the one
-    // from the root overlay.
-    expect(
-      ancestorRenderTheaters(tester.renderObject(find.byKey(menuItemKey))).single,
-      tester.renderObject(find.byWidget(rootOverlay)),
-    );
-  });
-
   // Regression test for https://github.com/flutter/flutter/issues/156572.
   testWidgets('Unattached MenuController does not throw when calling close', (WidgetTester tester) async {
     final MenuController controller = MenuController();