Ehem.. So what if someone was dumb enough to NOT read the instructions and definitely drag and dropped SOL into CM? How would said dummy fix this? Asking for a friend.
I actually went through this before haha. However, I found these to be the following solutions.
1.) Manually Delete the Files. This may sound intimidating initially, but it becomes a lot less once you look at the .7z archive Sol comes packaged in. It only modifies the following folders: apps, content, extension, and system.
What I did was that I had two file explorer windows open: one for my AC root folder and one with the Sol archive in my downloads. I went through each folder in the Sol archive to find the files it added and deleted their equivalent in the AC root folder.
*Also make sure to delete soluninstall.bat and description.jsgame and make sure to verify your game files just in case any base game, non-modded content was deleted or modified by chance.
2.) Delete apps, content, extension, and system folders entirely. I'd personally use this as a last resort as it's the most destructive. Completely delete these folders since these are what Sol modify, then verify your game files through Steam to have them restored, but this will not restore modded content, only Kunos ones.
Do note that this method will also delete mods that you have downloaded into these folders like cars, tracks, and pp filters. This will restore large portions of the game back to vanilla, so be warned.
3.) Run soluninstall.bat - this is a file that comes with Sol that you're supposed to put in the AC root folder. Run it as administrator and it will remove Sol from AC. However, if someone drag and dropped Sol and performed an invalid install, this might not be effective at removing the files. This is pretty good though if you have Sol installed properly but want to have a clean slate!
Hopefully, this was of use and let me know if you found better solutions than these three.