cmd/godoc: remove golang.org serving code

The code to serve the golang.org website has been moved to
the golang.org/x/website sub-repository. x/website has become
the canonical source of the golang.org website as of CL 162157,
and so this code can be removed from here now.

This has the benefit of removing some external dependencies
that were only used by the website in production mode, and
in turn enabling x/tools to be a smaller tools-related module.

In future changes, the golang.org/x/tools/cmd/godoc command
will be reduced in scope to be a tool for serving Go package
documentation only, not the rest of the golang.org website.

Run go mod tidy (using Go 1.12 RC 1).

Updates golang/go#29206
Updates golang/go#29981

Change-Id: I61fd25627d0506901b04688dea8d8c9da9fe8f04
Reviewed-on: https://go-review.googlesource.com/c/162400
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Channing Kimble-Brown <channing@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>