Flatten all threads in the comments

web/modules/custom/agaric_migration/migrations/agaric_comment.yml

@@ -6,16 +6,10 @@ source:
   constants:
     entity_type: node
     comment_type: comment
+    default_pid: 0
 process:
-  pid:
-    -
-      plugin: migration_lookup_with_zero
-      migration: agaric_comment
-      source: pid
-      no_stub: true
-    -
-      plugin: default_value
-      method: 0
+  # All the threads are going to be flatten.
+  pid: 'constants/default_pid'
   entity_id:
     -
       plugin: migration_lookup