Baking NavMesh for multiple scenes doesn't delete old NavMeshs

- Load two scenes with baked navmesh (for each scene separately) additively and bake NavMesh.
- NavMesh asset saves in a folder named as active scene and both scenes refference to it.
- Check NavMesh asset for not active scene. It still exists, but scene is not refference to it anymore.

By analogy to lightmaps this NavMesh should be deleted as redundant.
Attached project contains EditorTest, you can launch it using "Window->Editor Test Runner"