inputs: rabbitmq: fix optional exchange binding

Make hot_bunnies implementation behave the same as the bunny
implementation: optionally bind the input queue to the output exchange
if the "exchange" option is defined.  If it is not defined, some other
out-of-band process will need to setup the bindings.  Remove the
backward-compatibility reference, because this is actually a useful
feature to continue supporting.

LOGSTASH-1300