Remove MimirProvisioningTooManyActiveSeries alert

[pracucci]

What this PR does

I propose to remove MimirProvisioningTooManyActiveSeries warning alert. Why?

• The alert is not actionable. The fact that we have more than 1.6M series per ingester is not an issue per-se. We run production clusters with an higher average number of series and it's not an issue.
• We have per-ingester max series limit which is an hard stop on the max series in the ingester, and we also have the alert MimirIngesterReachingSeriesLimit: this alert triggers with warning severity when in-memory series reach the 80% of limit and with critical severity when reach the 90%.
• We have other limits triggering if ingester is under pressure, for example MimirAllocatingTooMuchMemory. In my opinion this is a better alert than just looking at the number of in-memory series.

Which issue(s) this PR fixes or relates to

N/A

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[56quarters]

This change makes sense to me. I can't remember the last time we (Grafana) alerted on this. I think the per-instance limit and alert makes a lot more sense than the average number of series.

[56quarters]

LGTM

[pracucci]

None shouted against this. I'm going to merge, but we can quickly revert if there are stronger opinions.