Standardise querying API calls count

[emyller]

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 (#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.

#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

[emyller]

Did some early investigation here with my buddy Claude, and using advice from @khvn26. I pulled the top 5 organisations (sorted by API usage) and compared the two functions. The following table will also have columns "Request count", for each function, which are omitted here.

Org	get_events_for_organisation	get_current_api_usage	Counts match
A	0.85s	0.81s	yes
B	0.77s	0.92s	yes
C	0.80s	1.04s	yes
D	0.87s	0.88s	yes
E	0.82s	0.82s	yes

This probably gives enough confidence to continue with the rollout, or maybe even go for 100% audience. Still, we'd need to clean up some code, update tests, etc — the diff would be a little too long to piggy back on #6975.