all: tidy std module

all: tidy std module

Run go mod tidy to remove go.sum lines that are unused as of CL 406914.
(This was spotted by the cmd/internal/moddeps.TestAllDependencies test.)

Change-Id: Ib0263465cd9559d793086e4d9322ea2c251a624a
Reviewed-on: https://go-review.googlesource.com/c/go/+/406897
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>