I started with Openhab 2.5 and configured few things by file.
Upgraded to OH 3 and all my things were imported. I tried to work now only with the GUI and deleted my Things from the folder.
But the old Things are still in my frontend locked, and I can’t remove them.
I have found that just removing the things file will not trigger openhab to remove the thing on my setup, too.
What you can do is generate an empty file with the old things file name, then it should work!
Are you sure that you deleted your files in the right folder? In my Installation, I had both etc/openhab/ and /etc/openhab2/ after migration to OH3.
Samba pointed still to openhab2, which took me some time to find out…