The fastest is to search through the jsondb files. On Linux
grep GalerieHelligkeit /var/lib/openhab/jsondb/*.json
That will tell you which files the Item appears in and perhaps enough right there to figure out which rule/link/etc. the Item still appears in. From there you can open the file with a text editor to search for the records where the Item name appears. That will give you all the information about where that Item is referenced. From there you should have enough information to navigate to it in MainUI and remove it. If not you can open the REST API docs under Developer Tools and search for that entry and delete it there.
As a last resort you can delete or edit the JSONDB directly but you must stop openHAB first and take extra special care not to mess up the syntax.
See OH 3 Tips and Tricks for a more detailed step by step description.