Mine disappeared when I reset my PS3 to factory default settings by accident - I had to re-input my login details for PSN, and then I could download them again, and play my save game as normal.
Assuming the game doesn't remember that you had them before, and re-apply the mods, that should work.
What exactly are you trying to remove??
Uninstall the game and reinstall it.
You'll have to re-download the cars.
It works for Chrome, it should for Stealth cars.