[skip changelog] Remove broken download stats workflow

[per1234]

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• [N/A] Tests for the changes have been added (for bug fixes / features)
• [N/A] Docs have been added / updated (for bug fixes / features)
• [N/A] UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Bug fix

• What is the current behavior?

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

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

On 2022-03-14, the runs of the "arduino-stats" GitHub Actions workflow began to fail:

Traceback (most recent call last):
  File "/home/runner/work/arduino-cli/arduino-cli/.github/tools/fetch_athena_stats.py", line 131, in <module>
    exec_id = execute(athena_client, query, DEST_S3_OUTPUT)
  File "/home/runner/work/arduino-cli/arduino-cli/.github/tools/fetch_athena_stats.py", line 27, in execute
    wait_for_query_execution_completion(client, execution_id)
  File "/home/runner/work/arduino-cli/arduino-cli/.github/tools/fetch_athena_stats.py", line [40](https://github.com/arduino/arduino-cli/runs/5539443430?check_suite_focus=true#step:4:40), in wait_for_query_execution_completion
    raise BaseException(
BaseException: query failed or canceled: INVALID_FUNCTION_ARGUMENT: URLDecoder: Illegal hex characters in escape (%) pattern - For input string: "b "

Because there had not been any relevant change in the repository between the last successful run and the first failing run, it seems likely that some external change caused the breakage.

• What is the new behavior?

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.

• Does this PR introduce a breaking change, and is
  titled accordingly?

No breaking change