Fix removing variants from product model

Fixes issue #2162

This is needed for when you remove a variant with
`Product::removeVariant` and then flush the changes.
Merge pull request #2164 from steffenbrem/fix-product-variants-removal

[BugFix] Doctrine mapping for removing variants from product model
Merge branch 'master' of https://github.com/Joypricecorp/Sylius