docs: remove stale release guide
docs: remove stale release guide

The release process has been in the `nomad-releases` private repo rather than
this one for quite a while now. Should be safe to remove this.

burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,hashicorp/nomad