Product attribute used as variant not completely removed.

Prereq.: Some attributes exist.

Steps:

1. Create Product Type and add attribute, set attribute as used for variants.
2. Create Product using this Product Type.
3. Generate some variants.
4. Edit the Product Type and remove the attribute used for variants.
5. Revisit the Product - Variants are removed.
6. Edit the Product Type and restore the attribute used for variants.
7. Revisit the Product.

Actual result: Variants are restored.
Expected result: Variants were removed permanently.