gddo-server: do not tee expected 404s

When a request is expected to 404 even after we redirect godoc.org to
pkg.go.dev, don't tee that request.

Change-Id: I1640bb2886ccdd55b2790f3b8d58b2d32eca8418
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/274699
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>