Drop support to ValueMetricType using cpu_memory_scaler

[JorTurFer]

Signed-off-by: jorturfer jorge_turrado@hotmail.es

This PR removes the support to Value metricType in CPU and Memory scalers. HPA doesn't support that metricType in these cases, so it doesn't make sense to maintain in as part of the scaler
#2200 (comment)

Checklist

• Commits are signed with Developer Certificate of Origin (DCO - learn more)
• Tests have been added
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable) Delete Value metricType from the CPU and Memory Scalers keda-docs#561
• Changelog has been updated

Fixes #2200

[zroubalik]

Could you please update the error message on L64 to something like fmt.Errorf("unsupported metric type, allowed values are 'Utilization' or 'AverageValue'")

[JorTurFer]

Could you please update the error message on L64 to something like fmt.Errorf("unsupported metric type, allowed values are 'Utilization' or 'AverageValue'")

Sure! Now it's more explicit :)