Remove shoppingcart default date values

The previous value means that those dates would have the value of
whenever the models were imported.

Reference TNL-3611

lms/djangoapps/shoppingcart/models.py

@@ -1190,7 +1190,7 @@ class CourseRegistrationCode(models.Model):
     code = models.CharField(max_length=32, db_index=True, unique=True)
     course_id = CourseKeyField(max_length=255, db_index=True)
     created_by = models.ForeignKey(User, related_name='created_by_user')
-    created_at = models.DateTimeField(default=datetime.now(pytz.utc))
+    created_at = models.DateTimeField(auto_now_add=True)
     order = models.ForeignKey(Order, db_index=True, null=True, related_name="purchase_order")
     mode_slug = models.CharField(max_length=100, null=True)
     is_valid = models.BooleanField(default=True)
@@ -1225,7 +1225,7 @@ class RegistrationCodeRedemption(models.Model):
     order = models.ForeignKey(Order, db_index=True, null=True)
     registration_code = models.ForeignKey(CourseRegistrationCode, db_index=True)
     redeemed_by = models.ForeignKey(User, db_index=True)
-    redeemed_at = models.DateTimeField(default=datetime.now(pytz.utc), null=True)
+    redeemed_at = models.DateTimeField(auto_now_add=True, null=True)
     course_enrollment = models.ForeignKey(CourseEnrollment, null=True)
 
     @classmethod
@@ -1300,7 +1300,7 @@ class Coupon(models.Model):
     course_id = CourseKeyField(max_length=255)
     percentage_discount = models.IntegerField(default=0)
     created_by = models.ForeignKey(User)
-    created_at = models.DateTimeField(default=datetime.now(pytz.utc))
+    created_at = models.DateTimeField(auto_now_add=True)
     is_active = models.BooleanField(default=True)
     expiration_date = models.DateTimeField(null=True, blank=True)