Subjects make sense for new discussions only, not in replies.

[srobotta]

Note: Please fill out all relevant sections and remove irrelevant ones.

🔀 Purpose of this PR:

• Fixes a bug
• Updates for a new Moodle version
• Adds a new feature of functionality
• Improves or enhances existing features
• Refactoring: restructures code for better performance or maintainability
• Testing: add missing or improve existing tests
• Miscellaneous: code cleaning (without functional changes), documentation, configuration, ...

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📝 Description:

Please describe the purpose of this PR in a few sentences.

When a new discussion is created, a subject must be set. In replies to that original post, the subject get's a prefix "Re:" however the content of the subject field is not displayed anywhere nor used when the response is added. Only for new posts the subject is taken to create a new discussion instance.
Therefore, do not show the user a field that has no relevance.

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📋 Checklist

Please confirm the following (check all that apply):

• I have phpunit and/or behat tests that cover my changes or additions.
• Code passes the code checker without errors and warnings.
• Code passes the moodle-ci/cd pipeline on all supported Moodle versions or the ones the plugin supports.
• Code does not have var_dump() or var_export or any other debugging statements (or commented out code) that
  should not appear on the productive branch.
• Code only uses language strings instead of hard-coded strings.
• If there are changes in the database: I updated/created the necessary upgrade steps in db/upgrade.php and
  updated the version.php.
• If there are changes in javascript: I build new .min files with the grunt amd command.
• If it is a Moodle update PR: I read the release notes, updated the version.php and the CHANGES.md.
  I ran all tests thoroughly checking for errors. I checked if bootstrap had any changes/deprecations that require
  changes in the plugins UI.

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🔍 Related Issues

None

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🧾📸🌐 Additional Information (like screenshots, documentation, links, etc.)

Before doing the change, I was checking how stackoverflow works. They also have a comment field for the reply but no further information.

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[TamaroWalter]

Thank you very much for this improvement and for changing the test files as well!

Kind regards