When an Attribute is removed from the Product Type, Variants created previously with that attrbiute cannot be updated.
Product Variant update fails with "Argument '$updateStruct' is invalid" error.

Steps to reproduce
1. Clean installation of Ibexa DXP Commerce v4.3
2. Create a new Attribute Group: "Test Attributes"
3. In "Test Attributes" create 3 Integer Attributes: "Attribute A", "Attribute B", "Attribute C"
4. Create a Product Type "Test Product Type" with default values, but use All 3 Attributes from "Test Attributes" group. Check "Used for product variants " for every one of them
5. Create a "Test Product" Product of "Test Product Type". Create variations using all 3 Attributes.
6. Remove "Attribute C" from "Test Product Type".
7. Go to "Test Product", edit one of the Product Variants.
8. Click on "Update".

Expected result
Product Variant is updated

Result
Error is thrown:

Argument '$updateStruct' is invalid: Attribute 'attribute_c' is not a variant discriminator