Merge Actor tool can no longer merge just materials for an actor

In 4.11 when you selected an actor with multiple material elements with the merge actors tool open you had the option to merge the material elements into a single one. This option is no longer valid in 4.12 and later since it will not give you the option unless you select multiple static meshes to merge.

This is a regression of behavior in 4.11 and is broken in 4.12 and a later.

1. Import the attached asset or open an asset with 2 or more material elements
2. Open the Merge Actor tool (Window > Developer Tools > Merge Actors)
3. Place the Static Mesh with multiple materials in the scene and select it.

Regression: YES

Results: Should be able to merge multiple material elements without need to select multiple static meshes.

Expected: Should merge just the materials for this single static mesh, or at least give the option.