app_agent_pool: Fix AMI/CLI AgentLogoff soft preventing agents from logging back in.

* Clear the deferred_logoff flag when an agent logs in.

(closes issue ASTERISK-22669)
Reported by: John Bigelow
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Merged revisions 400754 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@400759 f38db490-d61c-443f-a65b-d21fe96a405b