Do not minimize the supply dates interval

Since we use a configured supply period, we do not have to minimize the
interval on which to compute the supply.

issue11318
review366931003
(grafted from e78dd7305c95c6b752342ee4aba8ec3406b1ef19)

--HG--
branch : 6.0

purchase_request.py

@@ -213,7 +213,7 @@ class PurchaseRequest(metaclass=PoolMeta):
     @classmethod
     def get_supply_dates(cls, product, **pattern):
         """
-        Return the minimal interval of earliest supply dates for a product.
+        Return the interval of earliest supply dates for a product.
         """
         Date = Pool().get('ir.date')
 
@@ -228,11 +228,7 @@ class PurchaseRequest(metaclass=PoolMeta):
                 date=next_day)
             if (not min_date) or supply_date < min_date:
                 min_date = supply_date
-            if (not max_date):
-                max_date = next_supply_date
-            if supply_date > min_date and supply_date < max_date:
-                max_date = supply_date
-            if next_supply_date < max_date:
+            if (not max_date) or next_supply_date > max_date:
                 max_date = next_supply_date
 
         if not min_date: