Remove the subnet_update_dns_servers task list

It's no longer being used.

roles/openshift_openstack/tasks/subnet_update_dns_servers.yaml

@@ -1,9 +0,0 @@
----
-- name: Live update the subnet's DNS servers
-  os_subnet:
-    name: openshift-ansible-{{ stack_name }}-subnet
-    network_name: openshift-ansible-{{ stack_name }}-net
-    state: present
-    use_default_subnetpool: yes
-    dns_nameservers: "{{ [private_dns_server|default(public_dns_nameservers[0])]|union(public_dns_nameservers)|unique }}"
-  when: not provider_network