Current behaviour

When removing a backend usage and the limits of an application plan that refers to the backend at the same time, the Operator fails to reconcile the Product. 

Steps to reproduce:

• Create 2 Backends: Backend A and Backend B.
• Create 1 Product which contains usages to Backend A and Backend B as well as an Application plan with limits related to Backend B. See attachment.
• Wait for the Operator to sync all resources. 
• Update the Product CR as follows with a single update: 
  • remove `spec.backendUsages.backend_b_123`
  • set `spec.applicationPlans.plan_foo_bar.limits` to `[]` (an empty list)
• The Product CR will fail to sync with the following error: 

  "message": "Task failed SyncApplicationPlans: Error sync product [test_product_123] plan [plan_foo_bar]: Task failed SyncLimits: application plan [plan_foo_bar] delete limit: error calling 3scale system - reason: {\"status\":\"Not found\"} - code: 404"
  

Expected behaviour:
Be able to remove both references:

• backend usage
• limits of the application plan related to backend
  at the same time.

Dev Notes
Please see this comment