spdy: delete package

The SPDY package has known bugs (9 currently) and incompleteness.

Rather than fix and finish it, delete it. Browsers are removing SPDY
support anyway. The future is HTTP/2.

Anybody who wants the old x/net/spdy can get it from git revision
d175081 and vendor it.

Fixes golang/go#10547
Fixes golang/go#10545
Fixes golang/go#10544
Fixes golang/go#10543
Fixes golang/go#10542
Fixes golang/go#10540
Fixes golang/go#10539
Fixes golang/go#6037
Fixes golang/go#4295

Change-Id: I98ede7f8da00af64015d0f2784cadba33286a6a5
Reviewed-on: https://go-review.googlesource.com/9300
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: David Symonds <dsymonds@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Dave Day <djd@golang.org>