x/tools/cmd/getgo: deprecate and remove it

[hyangah]

The infrastructure to support this experimental tool hadn't been maintained. (get.golang.org)
We are now recommending golang.org/dl/* or, after go1.21, we have a better story.
Delete and remove it.

[findleyr]

I think there's still a need for a tool like cmd/getgo -- the instructions for linux at https://go.dev/doc/install are surprisingly manual. Could we instead fix it?

[hyangah]

Added "Proposal" label because there are some users depending on this.