How can I restore GTA V to how it was before I modded it?

There's really no way to fix it now unless you reinstall the game, but in the future, when installing mods to GTA V, follow these steps:

1. Open OpenIV and install the ASI Loader and such under 'tools > ASI Manager'. Make sure you install all 3 items by clicking "install" next to each one

2. When you go to edit any RPF or important file, or you go to replace files that were included with the clean game, right click on the item you are going to edit/replace and click "add to mods folder". This will put that file in the mods folder of GTA V, which will be loaded by OpenIV when you start the game. So basically when it launches the game, instead of getting car models from: Grand Theft Auto V/.../.../.../vehcles.rpf, it would get them from Grand Theft Auto V/mods/.../.../.../vehicles.rpf.

3. When you want to return to the clean GTA V, just go back into openIV, go back to 'tools > ASI Manager' and click uninstall for all 3. Also make sure to delete any external ASI loader like ScriptHookV or ScriptHookDotNet which will crash your game before entering online.

If you didn't edit any stock files (Ex, you just added files in the scripts folder and places like that), then deleting the added files would work to restore, but it sounds like you edited some stock GTA V files, so you would have to reinstall. Being a modder myself, the reinstalling gets tiring, but sometimes it has to be done!