https://github.com/flutter/engine/pull/40706 added a duplicate `NSEvent (KeyEquivalentMarker)` category. This PR removes it. It also removes the call to `markAsKeyEquivalent` from `FlutterViewController`. That call is internal to `FlutterTextInputPlugin` and classes outside should not be calling it. [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style