[Fullstack] Remove test_port_shut_down module

This fullstack module contains only one test test_port_shut_down and it
is now replaced by the API test in neutron_tempest_plugin (see depends
on patch).

Depends-On: https://review.opendev.org/c/openstack/neutron-tempest-plugin/+/902638
Change-Id: I79bdc4a48ddbdb7d5872ec9422e97445bf7fa553
(cherry picked from commit fea0e7d)

Author: slawqo