In gitlab_subscription_histories do not track the creation

As part of the Issue #13297 (moved) & MR !19694 (merged) we're tracking the creation and update and deletions of gitlab_subscription records.

Now retrospectively I think we don't need to track the 'creation'

• as the previous state is empty
• the values created_at aren't meant to change at all.

In gitlab_subscription_histories I think we don't need the historical tracking for creation.

• It was there for completeness but the previous state is empty
• and the creation date are already obtainable from the created_at even after an update and deletion of an existing gitlab_subscription row.

Here's an example for a previous state. It only shows the creation of the gitlab_subscription, which should be fetchable from the created_at in case of a deletion.

 id | gitlab_subscription_created_at | gitlab_subscription_updated_at | start_date | end_date | trial_ends_on | namespace_id | hosted_plan_id | max_seats_used | seats | tri
al | change_type | gitlab_subscription_id |          created_at           
----+--------------------------------+--------------------------------+------------+----------+---------------+--------------+----------------+----------------+-------+----
---+-------------+------------------------+-------------------------------
  1 |                                |                                |            |          |               |              |                |                |       |    
   |           0 |                3737036 | 2019-12-11 20:18:36.592653+00