[FIX] mrp_multi_level: allow to delete product.mrp.area records

Planned orders and mrp moves are mostly data that is regenerated every time
that the multi level mrp is run.

mrp_multi_level/models/mrp_inventory.py

@@ -30,6 +30,7 @@ class MrpInventory(models.Model):
         string="Product Parameters",
         index=True,
         required=True,
+        ondelete="cascade",
     )
     company_id = fields.Many2one(
         comodel_name="res.company",

mrp_multi_level/models/mrp_move.py

@@ -17,6 +17,7 @@ class MrpMove(models.Model):
         string="Product MRP Area",
         index=True,
         required=True,
+        ondelete="cascade",
     )
     mrp_area_id = fields.Many2one(
         comodel_name="mrp.area",

mrp_multi_level/models/mrp_planned_order.py

@@ -17,6 +17,7 @@ class MrpPlannedOrder(models.Model):
         string="Product MRP Area",
         index=True,
         required=True,
+        ondelete="cascade",
     )
     mrp_area_id = fields.Many2one(
         comodel_name="mrp.area",