ARI: Resolve a subscription leak against implicit bridge subscriptions

ARI: Resolve a subscription leak against implicit bridge subscriptions

When a channel in a stasis application is joined to a bridge, a subscription
for that bridge is created implicitly for the stasis application serving the
channel. Prior to this patch, subsequent removals of the channel from the
bridge would leave the subscription open.

Review: https://reviewboard.asterisk.org/r/3380/
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Merged revisions 411086 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@411090 65c4cc65-6c06-0410-ace0-fbb531ad65f3