Wrong image handling for simple products leads to duplicate images

[mmenozzi]

There's an issue handling images for simple products.

Here the ImageValueHandler associates the imported image to both product and product variants even for simple products (products with only one variant).

This is wrong because in the Sylius admin UI when the simple product is saved Sylius removes the relation between the image and the product variant.

Then, when the same product is imported again with this Akeneo plugin the image gets duplicated because the method ImageValueHandler::getExistentProductVariantImage() doesn't return any image because the relation with the variant does not exist anymore.