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MINOR: [Docs][Python] Remove outdated reference to libhdfs3 backend (#13443)

While the (deprecated) docs give an example of using libhdfs3 instead of the JNI interface, this actually hasn't existed in a long time.

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>