[FIX] purchase: Delivery Lead Time not working

Currently the delivery date on PO line will always be set to today if
there is no receipt date on PO, which makes the delivery lead time not
working. Fixed this issue in this commit.

Task 2357198

X-original-commit: 147aa4c67fb912a2e62dd8be1fc770db258e28d0

addons/purchase/models/purchase.py

@@ -1016,7 +1016,6 @@ class PurchaseOrderLine(models.Model):
             return
 
         # Reset date, price and quantity since _onchange_quantity will provide default values
-        self.date_planned = self.order_id.date_planned or self._convert_to_middle_of_day(datetime.today())
         self.price_unit = self.product_qty = 0.0
 
         self._product_id_change()
@@ -1071,7 +1070,7 @@ class PurchaseOrderLine(models.Model):
             params=params)
 
         if seller or not self.date_planned:
-            self.date_planned = self.order_id.date_planned or self._get_date_planned(seller).strftime(DEFAULT_SERVER_DATETIME_FORMAT)
+            self.date_planned = self._get_date_planned(seller).strftime(DEFAULT_SERVER_DATETIME_FORMAT)
 
         if not seller:
             self.price_unit = self.product_id.standard_price