I've long had this same issue where I'm getting notification of actions needed. I just deleted my old car, but all the entries are still there. I tried deleting them manually, and when I sync, they reappear.
Same thing for me. I am unable to get the notification icon to go away. I've tried deleting the app, re-installing, and it was immediately there...before I even restored the information. I've gone through the CSV and used it to delete any old information that was from a deleted vehicle, but that didn't resolve it.
Which by the way, as a suggested improvement, I'd offer the option to have all data removed when deleting a vehicle. I don't care about years worth of fuel data skewing my results when I don't own that vehicle anymore.
I should also add that turning off notifications within the Gas Cubby app or all together through the OS notification settings has no effect on it. The notification remains as if it's part of the app icon design.