Remove support for BibTeXML

[koppor]

Since more than a decade, we support BibTeX as XML as import and export format.

Although the code is clean, it seems that BibTeXML is not used by anyone - and even the original author says, it is dead. See Zearin/BibTeXML#1

This would mean to remove

• org.jabref.logic.importer.fileformat.BibTeXMLImporter
• org.jabref.logic.exporter.BibTeXMLExporter

and related classes / test files.

[sreenath-tm]

Hi, I would like to contribute to this issue. I have removed the mentioned files and the corresponding files and their references. Can you assign me this issue or can I directly raise the pull request?

[sreenath-tm]

I have raised the PR can you please take a look if it looks good?