rulers are never removed from the ring if evicted

[bboreham]

Basically the same problem as #1521, but rulers have no in-memory state to flush so it should be easier to ignore (or auto-forget) dead ones. Maybe we should worry a little about a split-brain state, but surely the worst case is doubling up evaluation which isn't fatal.

The error message is:

level=error ts=2020-01-31T09:13:05.403429734Z caller=ruler.go:362 msg="unable to verify rule group ownership ownership, will retry on the next poll" err="at least 1 live ingesters required, could only find 0"

[stale]

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