How do I delete columns that workflows create in a sharepoint list

Here is what you should do:

1. Get SharePoint Manager from Codeplex, if you've not already. It's a freeware with very useful feature for people like us. you can download it.

2. Run SharePoint Manager as Administrator 

3. Under the site -> List -> Fields, change the property of Workflow field Allow Deletion True

4. Open the site in SPD, Edit List coulmns, Show Read Only columns & finally delete.

Sharepoint Manager Screen:

Sharepoint Designer