bridge.c: Hangup attended transfer target if bridged

After completing an attended transfer the transfer target channel was not being
hung up after leaving the bridge. Added an explicit softhangup to hangup said
channel, but only if it was previously bridged.

ASTERISK-24782 #close
Reported by: John Bigelow

Change-Id: Idde9543d56842369384a5e8c00d72a22bbc39ada