Remove unused flavors cache functions

[knikolla]

Remove the following code block as it is no longer needed and check for other references to flavor cache.

https://github.com/CCI-MOC/openstack-billing-from-db/blob/7dfbc657d2e93db660b755fa5f6d63475ae57d44/src/openstack_billing_db/billing.py#L129-L142