Fixed, in an older release the device category was deleted by opening an popup, and the function wasn't in "utils.inc.php" library, but inside of that popup.
To remove the device category, you can either wait for next release, which will have the fix for this issue, or you can go to database and run a sql query DELETE FROM DeviceCategory WHERE PK_DeviceCategory=X;
with X the ID of category who is displayed next to category name in Device Templates page.