Remove generateModsSource (and mods-3-7.xsd)

[koppor]

Follow up to #9740. Finally fixes #9682.

Removes obsolete files.

@yanyanliu1400 and we have overlooked following statement in the issue description:

Similar to #9673, the gradle build target generateModsSource can be removed and the xsd file be removed from the source repository.

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1. Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
2. Tests created for changes (if applicable)
3. Manually tested changed features in running JabRef (always required)
4. Screenshots added in PR description (for UI changes)
5. Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
6. Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.