gddo-server: don't tee /_ah/* requests to pkg.go.dev

Currently, requests to App Engine admin paths at godoc.org/_ah/* are not
redirected to pkg.go.dev.

This change stops making a POST request to the teeproxy when a user
visits any of the admin paths.

Change-Id: I77f39b8289eab1086bc987adb218ee55ed48c919
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/235519
Reviewed-by: Julie Qiu <julie@golang.org>