Remove Preview feature Microsoft.ContainerService/CustomNodeConfigPreview #820

The Microsoft.ContainerService/CustomNodeConfigPreview was included in the Microsoft.ContainerService/managedClusters/agentPools module and should be removed/disabled until a strategy for preview features has been determined. This was introduced in #497 (Expand agent pool properties)

This is causing the error shown below:

22:40:12 - The deployment
| 'managedClusters-20211215T2212172885Z' failed with error(s).
| Showing 2 out of 2 error(s). Status Message: At least one
| resource deployment operation failed. Please list deployment
| operations for details. Please see
| https://aka.ms/DeployOperations for usage details. (Code:
| DeploymentFailed) - { "code":
| "PreviewFeatureNotRegistered", "message": "Preview feature
| Microsoft.ContainerService/CustomNodeConfigPreview not
| registered." } (Code:BadRequest) Status Message: At least
| one resource deployment operation failed. Please list
| deployment operations for details. Please see
| https://aka.ms/DeployOperations for usage details. (Code:
| DeploymentFailed) - { "code":
| "PreviewFeatureNotRegistered", "message": "Preview feature
| Microsoft.ContainerService/CustomNodeConfigPreview not
| registered." } (Code:BadRequest) CorrelationId:
| 1fc63799-a3bd-42c3-87b9-0089702fab28