Be sure to delete all files of the extension. Looking the error you are reporting it seems at least this 2 files are still present in your project
app/design/adminhtml/default/default/layout/eltrino/diamantedesk.xml
app/etc/modules/Eltrino_DiamanteDesk.xml
You can find here https://github.com/eltrino/DiamanteDeskMagentoExtension all extension files