Remove subnet from router on tf-elastx_cleanup (#8425)

The tf-elastx_cleanup test job was failed with error message:

Port xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx cannot be deleted
directly via the port API: has device owner network:router_interface.

That means necessary to remove a subnet from the router before
deleting the port.
This adds a method to removes a subnet from the router automatically.