gddo-server: do not redirect plain text requests

Pkg.go.dev doesn't support an plain text interface yet, but godoc.org
does: https://github.com/golang/gddo/wiki/Plain-Text-Interface

Do not redirect these requests until a decision is made about that
feature.

For golang/go#44029

Change-Id: I54466f2180db3b30a24299e49fc8fca107fe9f9d
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/288949
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Trust: Julie Qiu <julie@golang.org>