app_confbridge: Fix crash from receiving an AMI action after ConfBridge unloaded.

Unloading ConfBridge caused the next AMI action received to crash
Asterisk.

* Add the missing unregister of AMI action ConfbridgeSetSingleVideoSrc
when ConfBridge is unloaded.

(closes issue ASTERISK-20994)
Reported by: Jeremy Kister
Patches:
      jira_asterisk_20994_v11.patch (license #5621) patch uploaded by rmudgett
Tested by: Rusty Newton, Jeremy Kister
........

Merged revisions 381067 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@381069 65c4cc65-6c06-0410-ace0-fbb531ad65f3