Remove publication of a channel snapshot when the technology is set

This patch removes said publication for a few reasons:
(1) It is unnecessary. Association of the channel technology with a specific
channel is an implementation detail that should be assumed to "just happen",
and consumers of Stasis don't need to be informed about it.
(2) Publication of said message can now cause crashes, as the actual creation
of a channel in normal locations now stages its messages. As a result, things
that create dummy channels (such as the SIP RTP QOS unit test) and associate
them with a channel technology were now crashing, as the channel itself was
not known by Stasis.
........

Merged revisions 400460 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@400461 f38db490-d61c-443f-a65b-d21fe96a405b