On product delete, clean challenges.prudctIds if deleted product is whitelisted ?

[Tirodem]

To keep in mind for potential impacts on challege incrementation on paid order ( #248 )
Use case :

• I have X products
• I create a challenge with Y whitelisted products
• I delete one of the Y whitelisted products

=> What happens on the next paid order ? Do we search for a deleted product ?
Either it's without impact and we keep the entry in challenge.productIds, or it's problematic and we clean challenges on product deletion

[coyotte508]

Really easy to do