Standardise querying API calls count

We've had `influxdb_wrapper.get_events_for_organisation`, which served the purpose of obtaining usage from organisations in SaaS. Then we introduced `influxdb_wrapper.get_current_api_usage` (https://github.com/Flagsmith/flagsmith/pull/3340), which... seemingly serves the same purpose. Both their code paths lead to almost the same communication with InfluxDB, except for some aggregation in Influx vs in Python.

https://github.com/Flagsmith/flagsmith/pull/6975 has introduced a flag `get_current_api_usage_deprecated` to help transition SaaS users to standard `get_events_for_organisation`.

## Acceptance criteria

- [ ] 100% of users are standardised without issues.
- [ ] The feature flag `get_current_api_usage_deprecated` is safely deleted.
- [ ] Code is updated to dismiss `get_current_api_usage`