gddo-server: delete unused teeproxy code

Code used to tee traffic from godoc.org to the pkgsite teeproxy is
deleted, since requests are now teed directly to pkg.go.dev.

Change-Id: Icf89a310afec7cb225e5e88be6b1265fc4bee4f5
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/279037
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>