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ARROW-18397: [C++] Clear S3 region resolver client at S3 shutdown (#14718)

This should hopefully suppress a failed assertion on recent AWS SDK versions.

See https://github.com/aws/aws-sdk-cpp/issues/2204 for upstream issue report.

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>