Fix regression in OMB sending with queues enabled -- items never got dequeued, leadin...

Fix regression in OMB sending with queues enabled -- items never got dequeued, leading to ever-growing queue and big delays.

Success return code from omb_broadcast_message was dropped in commit ec88d2650ea4371cf53229171851747b31587e4b (Aug 10 2009) which switched us to libomb backend. With queues enabled, this would lead to the notice being readded to the outgoing OMB queue for redelivery as the queue system thought the send failed. The resends caused extra load and confusion for third-party sites, and more worryingly just plugged up our own queue so legit messages were badly delayed.

This commit should restore the previous state, where we fire-and-forget; that is, we're not actually checking to see if all remote subscribers received the message successfully and there will be no resends.