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)
........

Merged revisions 424181 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@424182 65c4cc65-6c06-0410-ace0-fbb531ad65f3