[purchase] Clean message on prices per supplier

- Task: https://www.odoo.com/web?#id=5152&view_type=form&model=project.task&menu_id=3942&action=327

- Pad: https://pad.odoo.com/p/r.1updBYpJroDlemrg

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Hello,

Rename cost price into "Supplier Price on product form or cost price" and remove value of "Supplier price on product form"

Merge both value cost price and supplier Price on product form in pricelist.
If not Supplier in product it's take cost price on product.

Thanks
Jaydeep Barot (JAR)
This code is a total mess. I know it comes from copy-pasting code from a few lines above, but this code keeps writing on the price variable until ... well, we run out of things to loop on. Is that behavior intended ?