Remove com.ddev.container-type from documentation (#791)

It is not needed any more to add this custom label to additional services
as ddev is using native com.docker.compose.service label

docs/users/extend/custom-compose-files.md

@@ -24,8 +24,6 @@ When defining additional services for your project, we recommended you follow th
 
   - `com.ddev.app-url: $DDEV_URL`
 
-  - `com.ddev.container-type: [servicename]`
-
 - Exposing ports for service: you can expose the port for a service to be accessible as `projectname.ddev.local:portNum` while your project is running. This is achieved by the following configurations for the container(s) being added:
 
   - Define only the internal port in the `ports` section for docker-compose. The `hostPort:containerPort` convention normally used to expose ports in docker should not be used here, since we are leveraging the ddev router to expose the ports.