[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)
Hello tde-banana-odoo
        I have reduce the code and remove unused code in _price_get_multi function.
Thanks.