net/http: update bundled x/net/http2

Updates bundled http2 to x/net git rev 63522dbf7

    http2: reduce allocations of (*clientConnReadLoop).handleReponse
    https://golang.org/cl/223783 (#37853)

    http2: remove unused errors
    https://golang.org/cl/220458

    http2: remove unused stream struct fields
    https://golang.org/cl/219857

    http2: fix typo in comment
    https://golang.org/cl/214602

    http2: workaround TCPConn CloseWrite not being supported on Plan 9
    https://golang.org/cl/209417 (#17906, #35904)

Change-Id: I0e48f32247938c3858170bf419624367d4faef4b
Reviewed-on: https://go-review.googlesource.com/c/go/+/224217
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>