chan_sip: Simplify some unref code by removing unlink_peer_from_tables.

ASTERISK-22945 #related
Reported by: ibercom
Patches:
  asterisk11-chan_sip-simplifies.patch uploaded by ibercom (License #6599)
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Merged revisions 424181 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 424182 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 424183 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 424184 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@424185 65c4cc65-6c06-0410-ace0-fbb531ad65f3