bridge_softmix: clear hold when joining a softmix bridge

bridge_softmix: clear hold when joining a softmix bridge

MOH continues to play to a channel if that channel was on hold prior to
entering a softmix bridge. MOH will not stop even if the original "holder"
attempts an unhold.

For the most part a softmix bridge ignores holds, so a participating channel
shouldn't join while on hold. This patch checks to see if the channel joining
the softmix bridge is currently on hold. If so then it indicates an unhold.

ASTERISK-28618

Change-Id: I66ccd4efc80f5b4c3dd68186b379eb442916392b