purchase_usability: remove unnecessary field `delivery_partner_id`

[clementmbr]

I understood this field was aimed to be used in PO reports, but there is not such report's inherited views in the modules. Moreover in native module purchase_stock there is already a condition displaying the warehouse's partner_id as Shipping Address (as expected) : https://github.com/odoo/odoo/blob/14.0/addons/purchase_stock/report/purchase_report_templates.xml#L11

So it looks like we can remove this unnecessary field delivery_partner_id.

cc @alexis-via @rvalyi

[rvalyi]

Hello @clementmbr ,

1. The fact that there is no report using the field in the module is not a big surprise: it's often the case with fields defined by the Custom Report Fields Wizard - @alexis-via - because we often need customized reports so maintaining a generic report is often not worth the effort, although this is somewhat done in https://github.com/akretion/odoo-py3o-report-templates (I would personally advocate for having equivalent reports as qweb now that qweb is solid enough).
2. I see a situation where the delivery_partner_id defined here could be useful and different from the native shipping address from Odoo CE https://github.com/odoo/odoo/blob/14.0/addons/purchase_stock/report/purchase_report_templates.xml#L11 , the case with a drop shipping address where you would deliver to the customer directly or some other non internal delivery address. The purchase_stock module only deals with a delivery address related to a warehouse. And the stock_drop_shipping module doesn't fix it.
3. I searched in the history to find out if it was a left over from an old module, but it is not th case: it appeared in v12 along with the purchase_stock module which was working very much like in v14 so I guess it was done for the reason I mentionned in 2.

[alexis-via]

I use that field in my py3o template reports, so I prefer to keep that field