fix: fix waiting to reach failureThreshold before fallback

[y-rabie]

The change removes a part that used to manually and immediately scale to Fallback.Replicas on encountering an error. Scaling to fallback is already done in GetMetricsWithFallback by recording an appropriate metric value the HPA will use, but only after Fallback.FailureThreshold failures.

The current existence of this part causes Fallback.FailureThreshold to be effectively unused.

doFallbackScaling and setFallbackCondition functions are removed since they are no longer used. The test TestScaleToFallbackReplicasWhenNotActiveAndIsError is no longer applicable I think

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