Remove DOI lookup from PdfContentImporter

[btut]

Identifiers found in PDFs will be fetched by the PdfMergeMetadataImporter (7947).
This way, we can fetch identifiers found by the other Pdf-Importers as well (Grobid, embedded-bib, XMP...)

• [] 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 documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.