[2.1.x] Removed unused fields from aggregation docs.

Backport of a1ffbfb8 from master

docs/topics/db/aggregation.txt

@@ -26,7 +26,6 @@ used to track the inventory for a series of online bookstores:
 
     class Publisher(models.Model):
         name = models.CharField(max_length=300)
-        num_awards = models.IntegerField()
 
     class Book(models.Model):
         name = models.CharField(max_length=300)
@@ -40,7 +39,6 @@ used to track the inventory for a series of online bookstores:
     class Store(models.Model):
         name = models.CharField(max_length=300)
         books = models.ManyToManyField(Book)
-        registered_users = models.PositiveIntegerField()
 
 Cheat sheet
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