remove almost all useless instances of add_blank_header() but one,

for which i need to investigate a bit more (but i am almost 100%
sure that one is also useless).
I do the commit in two steps so it is easier to track the problem.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@26915 65c4cc65-6c06-0410-ace0-fbb531ad65f3