don't iterate through all dialogs to find and delete old subscribes

don't iterate through all dialogs to find and delete old subscribes

On every incoming subscribe there is a iteration through all dialogs to find old subscribes and delete them. This is slow and not RFC conform. This was only needed in 1.2 cause a subscribe was not deleted when a dialog was destroyed, after 1.4 a subscribe get removed when its dialog is destroyed.

(closes issue #17950)
Reported by: schmidts
Tested by: schmidts

Review: https://reviewboard.asterisk.org/r/901/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@289622 65c4cc65-6c06-0410-ace0-fbb531ad65f3