http2: remove Transport.Fallback
http2: remove Transport.Fallback

We decided to glue together the HTTP/1 and HTTP/2 Transports in the
other direction, having the HTTP/1 code (net/http.Transport) start the
flow.

Remove the http2 Transport.Fallback for now, rather than leaving it
half implemented.

For background, see https://golang.org/cl/16090

Updates golang/go#6891

Change-Id: I511bc6d35a1a9a8e20010bd95ff694a894f42aa4
Reviewed-on: https://go-review.googlesource.com/16181
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

albertjin/golang-net,albertjin/golang-net,DramaFever/golang.org-x-net,cmars/tools,cmars/tools,cmars/tools,DramaFever/golang.org-x-net,cmars/tools,golang/net,cesanta/goxnet,cesanta/goxnet