internal/teeproxy: don't tee expected 404s
internal/teeproxy: don't tee expected 404s

Don't tee requests that we expect to 404 on pkg.go.dev, since this makes
the logs noisy and the information isn't useful.

Change-Id: I01217a2a99c943bb1e8639ee00ff0f00d29d53d1
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/257971
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite