Default to storage class being undefined

* This is so that users can intentially set it to an empty string if they want to use the default storage class
  * conversely, now users can manually create a pvc that does not utilize the default storage class

roles/backup/defaults/main.yml

@@ -10,6 +10,3 @@ backup_pvc_namespace: "{{ meta.namespace }}"
 
 # Size of backup PVC if created dynamically
 backup_storage_requirements: ''
-
-# Specify storage class to determine how to dynamically create PVC's with
-backup_storage_class: ''