[FIX] mrp: force empty values for fields that were dropped

This ensures the field values are cleared during
upgrades from previous versions
[FIX] product,purchase,point_of_sale: force empty values for dropped fields

At 96f038a product-related fields were removed due
to an important product.template/product.product
refactoring. As the field values were simply
dropped, they may not be nullified when upgrading an
existing database. Forcing them to False will take
care of it.