Remove TODO comment

According to [1] we should not relocate "wait_for_port_detach" method
under tempest/common libery, since only one test uses this method.

[1] https://review.openstack.org/#/c/468815/3

Change-Id: I725924d0dfd88939ff07d44995ed91884b6305bf
diff --git a/tempest/api/compute/servers/test_attach_interfaces.py b/tempest/api/compute/servers/test_attach_interfaces.py
index 6c5cc79..65d5042 100644
--- a/tempest/api/compute/servers/test_attach_interfaces.py
+++ b/tempest/api/compute/servers/test_attach_interfaces.py
@@ -49,7 +49,6 @@
         cls.subnets_client = cls.os_primary.subnets_client
         cls.ports_client = cls.os_primary.ports_client
 
-    # TODO(mriedem): move this into a common waiters utility module
     def wait_for_port_detach(self, port_id):
         """Waits for the port's device_id to be unset.