Don't unsubscribe from the AMI message router from manager_bridges

The AMI message router is owned wholly by manager.c. Previously, each of the
manager_{item} source files had their own message router and they unsubscribed
from each; once they moved over to using a single message router only a single
unsubscribe became necessary.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@396158 65c4cc65-6c06-0410-ace0-fbb531ad65f3