misc: Remove *_count fields

[vincent-pochet]

Context

The API response are returning a lot of counters. Since the exposed resources can be linked to a lot of objects and can be called a lot, these counters are posing a scalability problem and are slowing down the global performance of the API.

To remediate the situation the decision was made to remove all these fields.

Changes

This PR removes the following fields:

• BillableMetric
  • active_subscriptions_count
  • draft_invoices_count
  • plans_count
• Plan
  • active_subscriptions_count
  • draft_invoices_count
• Tax
  • add_ons_count
  • charges_count
  • customers_count
  • plans_count