@yamaro If you no longer have the authenticator nor the backup codes from the two steps verification, I'm afraid that you have lost access to your account. It means that you also will not be able to delete it.
If you have a PC with Sync enabled, go to Manage Bookmarks (Ctrl-Shift-B) and click on "Other speed dials". Delete the folder for the device in question, and the device will be removed from the server.
Bump. Why is it not possible to delete entries here?