Remove a useless statement

This statement is useless since both 'username' and 'password' are set to None
in the for loop, and that they are not used outside of the loop.

Removing this line also help us getting rid of a false positive thrown by
bandit.

Change-Id: I2aa1a16f30928b77aa40c5a900e35b7bf752658a

oslo_messaging/transport.py

@@ -381,7 +381,6 @@ class TransportURL(object):
 
         hosts = []
 
-        username = password = ''
         for host in url.netloc.split(','):
             if not host:
                 continue