test_stasis_endpoints: Remove expected channel snapshot

We no longer publish a channel snapshot when it is associated with an endpoint;
after all, the channel itself hasn't changed - the endpoint state has changed.
This updates the channel_messages unit test accordingly.
........

Merged revisions 416298 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@416299 65c4cc65-6c06-0410-ace0-fbb531ad65f3