Material Instance references to textures remains even when texture parameter removed from parent material

Removing texture parameter from parent material doesn't remove material Instance references to the default texture.

1. Create a new material that references a texture with a texture sample paramater
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2. Make an instance from that material
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3. Checking the referenced assets for the texture in the reference viewer should give a view like this:
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4. Delete the Texture Sample Parameter in the original Material and save it
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5. Notice that the instance parameter for the texture is no longer in the material instance
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6. But in the reference viewer, the reference from the material instance to the texture still remains
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7. Apply the Material instance to something in the level, and save the level. Select File > Cook content for Windows to cook the game data
8. Inspect the project's Saved\Cooked\WindowsNoEditor\ directory and observe that the texture is included in the cook, even though it is no longer used.
9. Even after restarting the editor, the reference remains