delete favicon.ico and robots.txt

The favicon.ico and robots.txt files have been with us in the root
directory since 2009 and 2011 respectively. Back then, the Go repo
had content for the golang.org website, which could be run locally.
Since these files were at the root of the website, they were added
to the corresponding location in the GOROOT tree—at the root.

In 2018, work started on factoring out golang.org website content
and code into a new golang.org/x/website repository (issue 29206).

The favicon.ico and robots.txt files were copied to x/website repo,
but some more work needed to be done before they would be picked up
and served when golangorg was executed in module mode. That work is
done by now (CL 293413 and CL 293414).

The scope of the godoc tool has also been reduced to just serving
Go package documentation and not the website (issue 32011), so it
can provide its own favicon.ico as needed (CL 300394).

This means these two files have no more use and can be deleted.
So long and goodbye!

Change-Id: Id71bdab6317c1dc481c9d85beaaac4b4eb92d379
Reviewed-on: https://go-review.googlesource.com/c/go/+/300549
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

robots.txt

@@ -1,2 +0,0 @@
-User-agent: *
-Disallow: /