Remove dead code and documentation for nonexistent feature.

multiplelogin was removed from chan_agent back in 1.6.0 when
AgentCallbackLogin() was removed.

(closes issue AST-948)
reported by Steve Pitts
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Merged revisions 373768 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 373769 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 373770 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@373771 65c4cc65-6c06-0410-ace0-fbb531ad65f3