Remove not needed validates_unique for route mappings (#4620)

Table `route_mappings` has a unique constraint for columns `app_guid`, `route_guid`, `process_type` and `app_port`.
Therefore we can rely on the DB to catch uniqueness errors and remove the sequel `validates_unique` in the model.
This reduces the number of DB queries as the following query is now skipped:

```SQL
SELECT 1 AS \"one\" FROM \"route_mappings\" WHERE ((\"app_guid\" = <app_guid>) AND (\"route_guid\" = <route_guid>) AND (\"process_type\" = 'web') AND (\"app_port\" = <port>)) LIMIT 1"
```

Author: johha

app/models/runtime/route_mapping_model.rb

@@ -34,7 +34,6 @@ def protocol_with_defaults
 
     def validate
       validates_presence [:app_port]
-      validates_unique %i[app_guid route_guid process_type app_port]
 
       validate_weight
     end
@@ -60,6 +59,15 @@ def has_app_port_specified?
       app_port != ProcessModel::NO_APP_PORT_SPECIFIED
     end
 
+    def around_save
+      yield
+    rescue Sequel::UniqueConstraintViolation => e
+      raise e unless e.message.include?('route_mappings_app_guid_route_guid_process_type_app_port_key')
+
+      errors.add(%i[app_guid route_guid process_type app_port], :unique)
+      raise validation_failed_error
+    end
+
     private
 
     def logger