Use DISTINCT instead of DINSTINCT ON for /v3/service_instances (#4210)

On large foundations using PostgreSQL DISTINCT ON might lead to wrong
query plans making the resulting execution very slow.

app/fetchers/service_instance_list_fetcher.rb

@@ -23,7 +23,7 @@ def fetch(message, omniscient: false, readable_spaces_dataset: nil, eager_loaded
 
         filter(dataset, message).
           select_all(:service_instances).
-          distinct(:service_instances__id)
+          distinct
       end
 
       private