Cleanup of deprecated agents

[Rotfuks]

Motivation

We rolled out Alloy as our main agent for logs and metrics thus unifying our agents in use to just one. Now we need to cleanup all other agents we no longer need. 

Stories

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1. Use self-service logging on Management cluster to dogfood our own solution #3715
   team/atlas +1
2. Get rid of Prometheus in agent mode
3. Get rid of promtail?
4. Get rid of Grafana agent?

Outcome

• We only have alloy in place saving us some complexity and potentially resources

[QuentinBisson]

@Rotfuks We discussed a bit this morning that we would prefer to keep the old agents for now (wait till they are deployed on big installations) before we remove the old ones to be sure they scale correctly. I would probably make this it's own epic (because the removal of the prometheus-operator is a bigger migration)

[Rotfuks]

Good point, I will see to it, thanks!