LandscapeSplineMeshesActor instances not deleted when World Partition is disabled

When landscape splines exist in a WP level, LandscapeSplineMeshesActor instances are created to allow streaming of splines. In a non-WP level, splines are components of the landscape actor. When a WP level has World Partition disabled, the LandscapeSplineMeshesActor instances remain, are not deleted, and other spline actors do not appear in the packaged version of the game.

1. Create a new level with world partition enabled.
2. Create a new landscape in that world, and add landscape splines to the landscape.
3. Run the 'Build Landscape Spline Meshes' command in the 'Build' menu for the level.
4. Observe that `LandscapeSplineMeshesActor` instances are created for the spline mesh.
5. Disable world partition for the level.
6. Observe that the `LandscapeSplineMeshesActor` instances remain.
7. Package the game.
8. Observe that only the `LandscapeSplineMeshesActor` instances appear in the packaged game, while the other landscape spline actors do not. Modifications made to other spline actors do not appear, only modification to the `LandscapeSplineMeshesActor` instance appear in the packaged game.