Remove unnecessary generation of informational cause frames

Remove unnecessary generation of informational cause frames

It is not necessary to generate information cause code frames on every
protocol event that occurs.  This removes all the instances where the
frame was not conveying a cause code and was instead just conveying a
protocol-specific message.  This also corrects the generation of the
message associated with disconnects for MFC/R2 to use the MFC/R2
specific text for the disconnect cause.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@369765 65c4cc65-6c06-0410-ace0-fbb531ad65f3