Remove GitHub Action workflow on `bin/setup`

[gazayas]

On the Bullet Train application I'm working on the GitHub Action which merges main to edge and pushes it to GitHub runs and then fails (because there's no edge branch), but if my understanding is correct, this is only necessary for the starter repository itself and not actual Bullet Train apps (we don't automatically make an edge branch when creating a new app so git has nothing to push to, and I think our developers will only need to run their tests against the gems which have been officially released).

I think we can easily delete the file in the bin/setup script.