sg: move to own repository, delete all remaining files
Created by: mrnugget
sg has moved to its own repository in the last sg hacking hour: github.com/sourcegraph/sg.
I transferred all the issues from this repository here to the sg repository. CI workflows etc. have also been setup over there.
This PR deletes the remaining files, updates references to the ./dev/sg folder, adds a note to the ./dev/sg/README.md that it has moved.
New, temporary install instructions
But that should only be the case for a short period of time. The goal is to have a single command that everybody can use to download/install the newest sg, see ticket https://github.com/sourcegraph/sg/issues/3