remove debug log statement in EXECUTOR shutdown callback

[psnelgrove-r7]

Using the signals module the library tries to gracefully shutdown the ThreadPoolExecutor with a callback.

For us this causes some issues with the logger module warning that you're writing to write to a log file after the file is closed. With these signal callbacks you can't control the shutdown order.

Traceback (most recent call last):
  File "/Users/psnelgrove/.local/share/uv/python/cpython-3.10.15-macos-aarch64-none/lib/python3.10/logging/__init__.py", line 1103, in emit
    stream.write(msg + self.terminator)
ValueError: I/O operation on closed file.
Call stack:
  File "/Users/psnelgrove/code/.../.venv/lib/python3.10/site-packages/alibabacloud_credentials/provider/refreshable.py", line 27, in _shutdown_handler
    log.debug("Shutting down executor...")
Message: 'Shutting down executor...'
Arguments: ()

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[psnelgrove-r7]

I also think this aligns with the feedback in #67