Updates golang.org/x/net/http2/hpack to x/net git rev 4dbf598 for:

   http2/hpack: remove unused pair function from package
   https://golang.org/cl/43851

   http2/hpack: remove pair function and initialize directly
   https://golang.org/cl/43190

   http2/hpack: move initialization to a static table
   https://golang.org/cl/43090

Updates #6853

Change-Id: I933a094623143c7333a0da7867d5d2e41da05234
Reviewed-on: https://go-review.googlesource.com/44007
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>