Steps to reproduce:

1. Download and import attached project.
2. Open "main" Scene.
3. Select "test" GameObject in the Hierarchy view.
4. Open Animator window.
5. Select a Transition or State and press Delete button.

Expected result: Object is deleted and no message box appears.
Actual result: A hindrance message box appears. (DeleteMessage.png)

Since we now have working undo mechanism in the Animator window, this message is unnecessary and slows down work with Animator. (For example: objects in Hierarchy view are deleted without any message)

Reproduced on: 5.4.0a5, 5.3.0f3. 5.3.0f1, 5.2.2f1, 5.1.4f1