[*] Project : Orders should rely only on ps_order_cart_rule

The goal of this PR is to ensure that we only rely on ps_order_cart_rule to generate Invoices, OrderSlip and every Order related PDF documents.
**Thoses changes have absolutely no impact on the PDFs generated. This is just some code cleanup and minor refactoring.**

We carefully looked out for any code part where the LEFT JOIN on ps_cart_rule would be needed, and the only place where that was the case is OrderSlip. OrderSlip used data from ps_cart_rule (reduction_percent and reduction_amount) to build a $products array with a 'product_price' index. However, this particular index is never used in the OderSlip so we're better off without it.

Last, but not least, ps_order_cart_rule has a free_shipping column, but it was never correctly filled because the free_shipping attribute wasn't declared in OrderCartRule.php. This is fixed as well.

Regards,