How did you overclock it in the first place? If you used software like MSI Afterburner / EVGA Precision then you changed nothing in the card. Simply disabling your overclock, or not running the software, would have the card running it's defaults.
If you wrote a new BIOS file to the GPU, then the settings would have been permanent until the original BIOS was flashed again.
A system restore point affects software and Windows primarily. Generally won't change an overclock program, unless the restore point doesn't include it, in which case it would be uninstalled, but you can do that without a system restore.