[BUG] PIM: RoleManagementPolicies_Delete defined the spec but not supported

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/authorization/resource-manager/Microsoft.Authorization/stable/2020-10-01/RoleManagementPolicy.json#L142

### API Spec version

2020-10-01

### Describe the bug

At the link above, the spec defines the RoleManagementPolicies_Delete operation. There's even an example. However, trying to use it, I only get `Bad Request("Not implemented exception")`.

### Expected behavior

Either the Role Management Policy is deleted, or delete it not defined in the spec.

### Actual behavior

The spec defines the delete operation but fails at run time with "Not implemented exception".

### Reproduction Steps

az rest --method delete --url 'https://management.azure.com/subscriptions/$SUB/providers/Microsoft.Authorization/roleManagementPolicies/8311e382-0749-4cb8-b61a-304f252e45ec?api-version=2020-10-01'

### Environment

_No response_