Switched subscription_id to use a standard custom_flatten

Following #12939 this will trigger proper generation of post-create code

Author: melinath

mmv1/products/bigqueryanalyticshub/ListingSubscription.yaml

@@ -30,7 +30,6 @@ immutable: true
 import_format:
   - 'projects/{{project}}/locations/{{location}}/subscriptions/{{subscription_id}}'
 custom_code:
-  post_create: templates/terraform/post_create/analytics_hub_subscription.go.tmpl
   pre_read: 'templates/terraform/pre_read/bigqueryanalyticshub_listing_subscription.tmpl'
   pre_delete: 'templates/terraform/pre_read/bigqueryanalyticshub_listing_subscription.tmpl'
   post_import: templates/terraform/post_import/analytics_hub_subscription.go.tmpl
@@ -122,7 +121,7 @@ properties:
     description: |-
       The subscription id used to reference the subscription.
     output: true
-    ignore_read: true
+    custom_flatten: 'templates/terraform/custom_flatten/id_from_name.tmpl'
   - name: 'creationTime'
     type: Time
     description: |-