Remove unused org.opendaylight.controller.xml.codec package/classes

Remove unused org.opendaylight.controller.xml.codec package/classes

The org.opendaylight.controller.xml.codec utility classes in
sal-clustering-commons are no longer used so remove them. They were
originally used by the remote RPC code for conversion of the legacy
CompositeNode class which has since been removed. The code referencing
CompositeNode et al was removed from these classes but unused
remnants were left behind.

Change-Id: Ic8f880c8075f076549bb822f6dfbaaad81595ed1
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>