Drop InfluxDB support

I noticed that we still had the manage.py telegraf command, originally used to generate data to be sent to an InfluxDB database to display some stats about Arkindex in Grafana. Grafana and InfluxDB have been removed from the dev architecture a while ago, and there have not been any talks AFAIK about doing those statistics again; generating them uses too much resources on the prod database and isn't really worth it. We can remove the command, and drop support for InfluxDB.