quantumclient.common.serializer module cleanup

Remove unused methods in quantumclient.common.serializer.XMLDeserializer
that tries to access non existing attributes of xml node:
* find_first_child_named
* extract_text
* find_children_named

Change-Id: I0aff5933fa75e50748e9d0325d898c2f6836fa58
Fixes: bug #1132850
Merge "quantumclient.common.serializer module cleanup"