ci: remove ZH dispatch from JA translation cron

[qiancai]

What is changed, added or deleted? (Required)

Purpose:

This avoids duplicate ZH translation runs because translation-zh.yaml already has its own scheduled trigger.

Changes:

• Remove the dispatch-translation-zh job from translation-cron.yml so the JA translation cron no longer dispatches the ZH translation workflow.
• Rename the workflow to Translation Cron JA and clarify the JA-related schedule and matrix comments.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

What is the related PR or file link(s)?

• This PR is translated from: N/A
• Other reference link(s): translation-zh.yaml already schedules ZH translation directly.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

Validation

• git diff --check -- .github/workflows/translation-cron.yml
• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/translation-cron.yml"); puts "YAML ok"'

[ti-chi-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[gemini-code-assist]

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[ti-chi-bot]

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