[http3client] get rid of tunnel API
[http3client] get rid of tunnel API

cwyang/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,cwyang/h2o,h2o/h2o,h2o/h2o,h2o/h2o,yannick/h2o,cwyang/h2o,devnexen/h2o,yannick/h2o,h2o/h2o,yannick/h2o,yannick/h2o,yannick/h2o,h2o/h2o,devnexen/h2o,h2o/h2o,h2o/h2o,yannick/h2o,yannick/h2o,cwyang/h2o,devnexen/h2o,cwyang/h2o,cwyang/h2o,yannick/h2o,devnexen/h2o,cwyang/h2o,devnexen/h2o,devnexen/h2o,h2o/h2o,devnexen/h2o,yannick/h2o,cwyang/h2o