Remove reference to editor assets in cooked build from landscape visibility layer

As soon as the visibility tool is used, we're adding references to the  "/Engine/EditorLandscapeResources/DataLayer" asset from landscape components.

If the user decides to cook without editor data, we ignore all assets whose paths starts with "/Engine/Editor". Warnings will be outputted, but the map can run without issue.

There are a few possible solutions to remove the warnings:

1) Remove reference to this layer during cook

2) Rename this layer to something else (/Engine/LandscapeResources/) so that it ain't treated as editor data anymore. A solution will be needed for existing data.

• Create a new map
• Create a landscape
• Apply a material that support masking
• Paint using the visibility tool so that a visibility layer is added
• Cook map without editor data (-SkipCookingEditorContent?)

Will result in the following warnings during cook:

LogSavePackage: Warning: /Game/Level/Map/Terrain/Island_Terrain imported Serialize:/Engine/EditorLandscapeResources/DataLayer.DataLayer, but it was never saved as an export.
LogSavePackage: Warning: /Game/Level/Map/Terrain/Island_Terrain imported Serialize:/Engine/EditorLandscapeResources/DataLayer, but it was never saved as an export.