[FIX] mrp: Unrelated product_template and product_product on the same BOM

Before this fix, we can create a Bom with both product_id and
product_tmpl_id filled where the product_id is not a variant of the
product_tmpl_id. This is unlogical and will be even trickier when the
user is not able to see the variant field on the BOM.

This fix puts the product_id back to False when the product_tmpl_id
is changed.