[Broker-J] Use context variable for configuring queue default alternate binding and remove references to x-qpid-dlq-enabled from docs

In Qpid 7.0 the way that DLQs were configured was changed, and the implementation simplified (see QPID-7606), however documentation (and code) was left over for the now non-functional x-qpid-dlq-enabled queue declare argument. Attempting to use this argument when declaring a queue on v7.0 or above now causes a cryptic error (unless suitable node creation policies are in place).