cmd/releasebot: delete

By now we've started using relui for all Go releases, and don't expect
to need releasebot anymore. It's not being maintained to keep up with
changes to the release mechanics, and some of the early communication
tasks integrated into it are getting in the way of API simplifications.

So, delete it.

We thank releasebot for its service over the years and many Go releases
it helped us successfully issue.

For golang/go#40279.

Change-Id: I405820f95315f1270534f5c188e3e6aa68ebd361
Reviewed-on: https://go-review.googlesource.com/c/build/+/419431
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
Reviewed-by: Jenny Rakoczy <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>