Static Meshes that are reimported with less materials can lead to map check errors on instances that had overriden materials that require manual cleanup

StaticMeshActors placed in the world can override the default materials of a static mesh. If the static mesh is then reimported with less materials, the actors maintain references to unused material overrides. These references can cause otherwise unused materials to be included in a cook.

The static mesh import dialogue does not offer a way to remap or otherwise remove these unwanted material overrides and resaving WorldPartition actors does not remedy the issue either.

Easiest repro (no reimporting meshes required):
1. Load Expanse level in Lyra project.
2. Enable a world partition region covering the level, so objects get loaded.
3. Run Map Check.
4. With this CL, these can be cleaned up by right clicking on one of the "More overridden materials..." map check errors, choosing "Select actors by message" to select all, then pressing the yellow warning triangle icon button that appears on the Static Mesh property in the editor.
5. The warning icon will disappear, and if Map Check is run again, the errors should be gone.

Original repro:
Create a static mesh with more than one material slot.
Place an instance of this mesh in the world.
Override all materials.
Reimport the mesh with less materials.
Run MapCheck and observe the warnings.