[camera_avfoundation] Remove outdated TODO comment in messages.dart (#11236)

The PR removes an outdated `TODO` code comment in the Pigeon `messages.dart` in `camera_avfoundation`.

@stuartmorgan-g - You already addressed it in 2024 here: 3584ddb#diff-ce4d13732a822cf36d7a1c574215e7974670567cd5667e2a1665c88ac5dc2615R163

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[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Author: cdeil

packages/camera/camera_avfoundation/pigeons/messages.dart

@@ -190,9 +190,6 @@ class PlatformSize {
 @HostApi()
 abstract class CameraApi {
   /// Returns the list of available cameras.
-  // TODO(stuartmorgan): Make the generic type non-nullable once supported.
-  // https://github.com/flutter/flutter/issues/97848
-  // The consuming code treats it as non-nullable.
   @async
   @ObjCSelector('availableCamerasWithCompletion')
   List<PlatformCameraDescription> getAvailableCameras();