I had the Harnessless Harness mod for a while, but decided to uninstall it. However, when I deactivated the mod, there was still a texture folder, so I deleted it thinking that the game would go back to using the default texture. Unfortunately, that didn't work; while I have the harness itself back, the body underneath now has a large nipble on the lower chest/upper stomach, and I don't know how to restore the default texture. What would I need to do?

Undo the thing you did? So.. do it again, but in reverse. Or if you have no idea what you did then.. delete everything and start over and pay attention in the future.

Or maybe ask the person who made the mod, or other people who use the same mod, on a forum.. for mods.

You could try "Verify cache" in Steam, it should check your files and re-download anything that is missing

It's unlikely that anything's missing, and much more likely that there are additional items that that won't do anything about.