fix: remove leftover line from migration file

Author: Derik Evangelista

db/migrations/20201013153721_add_service_instance_constraint_to_service_bindings.rb

@@ -9,7 +9,6 @@
 
     alter_table :service_bindings do
       add_unique_constraint [:service_instance_guid, :app_guid], name: :unique_service_binding_service_instance_guid_app_guid
-      add_unique_constraint [:app_id, :route_id, :app_port], name: :apps_routes_app_id_route_id_app_port_key
     end
   end