[FW][FIX] crm: remove res_model/res_id from meetings when unlinking leads

With Calendar and CRM installed:
- Create a lead and a meeting related to that lead.
- Delete the lead.
- Go to the meeting form view from the calendar App.
- Click the Document action button and nothing will take place.

After this commit if you delete the lead, you will no longer be able to the
see the button as document information is correctly reset.

Task-2917174
Closes odoo/odoo#42450
Closes odoo/odoo#47497

closes odoo/odoo#96190

X-original-commit: 71ccb7dae43a13521f2a290ce76cafd8fb5b963f
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>