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: http://svn.asterisk.org/svn/asterisk/trunk@396158 f38db490-d61c-443f-a65b-d21fe96a405b