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ARROW-12419: [Java] Remove to download flatc binary for s390x

This PR is a follow-up of #10058. #10058 avoids executing flatc during the Java build process by statically generating Java files from the schema. Now, flatc is not necessary.

Since flatc for s390x is explicitly downloaded in a script, we can drop this download.

Closes #10067 from kiszk/ARROW-12419

Authored-by: Kazuaki Ishizaki <ishizaki@jp.ibm.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>