Remove BibTeXML support

[sreenath-tm]

This fixes #9540 where the importer and exporter for BibTexXML have been removed and the corresponding test classes also have been removed or modified accordingly.

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• 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.

[Siedlerchr]

Looks good so far, please check also in the resources folder src/test/resources/logic for importer and exporter in the fileformat subfolder and remove them as well:

Please also add a changelog entry under Removed

e.g.:

[sreenath-tm]

All the files with BibTeXML prefix will not be useful now right. The files having prefix should be removed from both importer and exporter right.

[Siedlerchr]

Yes, remove them please

[sreenath-tm]

Hi, @Siedlerchr I have removed the BibTeXML-related resources from both the importer and exporter. The changes to the Changelog.md have also been done.

[Siedlerchr]

Thanks again for your contribution and your ongoing interest in JabREf

[koppor]

While investigating #9673, I learned that in the context of importer/exporter adjustment, maybe, build.gradle needs to be changed.

@sreenath-tm I needed to do a follow-up PR on this (#9684). There were some left-overs. Do you know the IntelliJ shortcut Ctrl+Shift+F? With that, one can search for all occurrences of text.

[sreenath-tm]

Yes, @koppor I had removed all the occurrences that I could find. Sorry If I have missed anything.

[koppor]

No worries, just wanted to give the hint of full text search (which I also forget about know and then...). Looking forward for other contributions by you 🤩.