Remove remnant of snapshot blob JSON types

Remove usage of the once-mandatory snapshot blob type field, refactor
confbridge stasis messages accordingly, and remove
ast_bridge_blob_json_type().

Review: https://reviewboard.asterisk.org/r/2575/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@390250 65c4cc65-6c06-0410-ace0-fbb531ad65f3