Revert "Fix crash from alt-tab'ing just after startup (#102453)" (#103074)

This reverts commit 74e9a6d9.

packages/flutter/lib/src/rendering/mouse_tracker.dart

@@ -319,8 +319,7 @@ class MouseTracker extends ChangeNotifier {
         // so that [mouseIsConnected], which is decided by `_mouseStates`, is
         // correct during the callbacks.
         if (existingState == null) {
-          if (event is PointerRemovedEvent)
-            return;
+          assert(event is! PointerRemovedEvent);
           _mouseStates[device] = _MouseState(initialEvent: event);
         } else {
           assert(event is! PointerAddedEvent);

packages/flutter/test/rendering/mouse_tracker_test.dart

@@ -133,16 +133,6 @@ void main() {
     listenerLogs.clear();
   });
 
-  // Regression test for https://github.com/flutter/flutter/issues/90838
-  test('should not crash if the first event is a Removed event', () {
-    final List<PointerEvent> events = <PointerEvent>[];
-    _setUpWithOneAnnotation(logEvents: events);
-    ui.window.onPointerDataPacket!(ui.PointerDataPacket(data: <ui.PointerData>[
-      _pointerData(PointerChange.remove, Offset.zero),
-    ]));
-    events.clear();
-  });
-
   test('should correctly handle multiple devices', () {
     final List<PointerEvent> events = <PointerEvent>[];
     setUpWithOneAnnotation(logEvents: events);