Skeletal Meshes keep reference to unused Material IDs

When importing a FBX with multiple materials and some of them are unused from their setup, they will still be referenced by the skeletal mesh in the Details Panel in the level editor, but in the Persona the skeletal mesh will only show the assigned materials.

As an example, if you create a Multi-Sub-Object material in 3Ds Max with 5 material IDs. When you assign this to a cube and only reference two of these material IDs on that mesh and import it, the Persona will only show the two materials assigned, but in the level details panel for the skeletal mesh there will be a list of all 5 materials referenced.

Tested in:
Broken: 4.12.5 CL-3039270
Broken: 4.13 CL-3106072
4.14 Dev-Editor CL-3116057

1. Create an asset in 3Ds Max with multi-sub-object material with more than 3, but only assign the 1st and 3rd material to the cube (or use the attached Max file/FBX asset)
2. Import into UE4 as a Skeletal Mesh
3. Open the Skel Mesh in Person and take note of the number of Material IDs
4. Place the Skel Mesh in the level and look at the Level Details for material IDs referenced

Regression: No

Results: The skeletal Mesh in the levels detail panel references all material elements regardless of what is actually assigned and showing in Persona.

Expected: The skeletal mesh should only reference the material IDs that are assigned to the mesh.