Remove unused GitHub release download stats workflow

The "github-stats" GitHub Actions workflow periodically gathers GitHub release asset download statistics for Arduino CLI
and pushes the results to Datadog.

There are no known problems with this workflow. However, the companion "arduino-stats" workflow that did the same for
the downloads of Arduino IDE from downloads.arduino.cc was broken and thus removed from the repository.

The GitHub stats are not very valuable on their own as they only provide an unknown fraction of the total downloads of
Arduino IDE. They have also not ended up being used.

The workflow also uses deprecated Node.js 12 runtime, which currently results in warnings printed to the workflow run
summary page, but will eventually cause the complete breakage of the workflow.

Since it doesn't provide any value and represents a maintenance burden, the workflow is hereby removed from the
repository.

Author: per1234