Remove broken download stats workflow

The "arduino-stats" GitHub Actions workflow was designed to periodically gather download statistics from Arduino CDN and
push results to Datadog.

The recorded stats from the identical system in the Arduino CLI repository showed a periodic decrease in total download
count. Since this is patently impossible, it is clear that something is wrong with the system and that the recorded data
is not trustworthy. An investigation into the problem
was never done.

On 2022-03-14, the runs of the "arduino-stats" GitHub Actions workflow began to fail. Because there had not been any
relevant change in the repository between the last successful run and the first failing run, it seems that some external
change caused the breakage.

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

Since the workflow was not ever working successfully and the lack of an investigation about that indicates that the
stats are not of immediate importance, the best course of action is to simply remove the broken infrastructure from the
repository rather than investing time into fixing something that isn't being used anyway.