internal/frontend: don't precheck 404s
internal/frontend: don't precheck 404s

Previously when a user received a 404, we checked all the candidate
module paths to determine if the path was recently fetched. If so, the
user is not presented with a Request button. This was done to create a
nicer user experience, but it also can result in a lot of queries when
we are being scraped. This check is removed to reduce queries in that
case.

Change-Id: If8917b5ddf6374fe17a742bd8d255fc6ced89e94
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/270266
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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