internal/frontend: delete redirect for "/mod/std" in serveDetails
internal/frontend: delete redirect for "/mod/std" in serveDetails

All requests to /mod/<path> are already being redirected to /<path>.

Change-Id: Id1ee029cfda884e6da65e852e50fc8a48dd6cb95
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/270597
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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