Remove mapping from product collection to generic Collection Entity

[jeroendk]

Remove mapping to Classification\Collection to stop forcing people to add an extra property to their generic Collection class.

I am targeting this branch, because (I think) this is a bugfix. not sure though... Some people might actually rely on the association being there? For our use case, it is a bug.

Another option would be to check if the productCollection property exists in the Collection class and if so add the association? We could remove the association altogether in a new major? I would like one's input on this.

Closes #644

Changelog

### Fixed
- Only add association mapping from ProductCollection to Collection when property exists
- Only add inversed side definition in ProductionCollection mapping when property exists. 

[greg0ire]

Thanks @jeroendk !

[jeroendk]

@greg0ire What is your release cycle for the Ecommerce bundle / when can I expect a release with this fix?

[wbloszyk]

@jeroendk
Can u look at this sandbox#620 ?
Also can you check the same thing like here with User-Customers relation?

[jeroendk]

@wbloszyk sure, where can I find the User-Customers relation (discussion)?

[greg0ire]

@greg0ire What is your release cycle for the Ecommerce bundle / when can I expect a release with this fix?

People like you ask me and I release. Expect one for tonight.

[jeroendk]

People like you ask me and I release. Expect one for tonight.

Awesome, thanks!