Remove the legacy "default_receiver" config from Cloud Monitoring

[ocofaigh]

The concept of a "default_receiver" (which we control using the enable_platform_metrics boolean) for a Cloud Monitoring instance is getting replaced by metrics routing instead (which is controlled by enable_metrics_routing_to_cloud_monitoring)

Once we get confirmation from the Observability team that all IBM Cloud services have migrated to the new approach, and once there is an official announcement from IBM Cloud on the deprecation of "default_receiver", we should remove it from our module and DA.

NOTE: enable_platform_metrics is exposed in many DAs as a virtual input, so all of them would need to be removed.