Disable autoprune by default.

(closes issue #13411)
Reported by: caio1982
Patches:
      res_jabber_autoprune1.diff uploaded by caio1982 (license 22)
Tested by: caio1982

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@142280 65c4cc65-6c06-0410-ace0-fbb531ad65f3