Remove dead tracking code for add-on purchase from SubscriptionsController

Overview

As part of https://gitlab.com/gitlab-org/customers-gitlab-com/-/issues/4436+ and CI min subscriptions have subscription manageme... (#4428 - closed), Compute Minutes and Storage purchase will always go through POST request (create action) regardless whether a new subscription is created or an existing subscription is amended.

As a result, the tracking code for Compute Minutes and Storage in update action is no longer executed.

Proposal

Remove the unused tracking code for Compute Minutes and Storage and ensure existing functionality is unaffected.

The unused code also has the only usages of track_subscription_management_event_deprecated method which can also be removed as part of this clean up.