skeletal mesh cannot use async scene

Since the option to use Async Scene with Mesh and CapsuleComponent appears under "Physics" in the Details Panel of a skeletal mesh, the ability to select these options is expected. Currently both boxes remain "grayed out" when the skeletal mesh is selected so that use Async Scene cannot be applied.

Regression: In UE4.14 the the option to Use Async Scene with the Capsule Component was available.

1. In UE4.15, open the Third Person Blueprint tempate
2. In the Viewport, select the Third Person Character
3. In the Third Person Character's Detail Panel search "Async" and view results under the "Physics" and "Collision"
4. Repeat the steps above in UE4.14 and compare
   RESULTS: In 4.14, under "Physics" you can select the the Capsule Component to use Async Scene. In 4.15 you cannot -Regression
   EXPECTED: In both 4.14 and 4.15 to be able to select "Mesh" and "Component" to use Async Scene or if not possible, the option not to appear in the Details Panel.

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