You can delete a CSV or API column from Pool from Pool >> Home >> Columns button at the top right. There is no undo and the data will be gone forever, so please be careful with this action. Btw, you can't delete screener question columns as those are tied to individual screeners.
Scroll down to Pool Columns and look for the rename and delete icons:
Keep in mind this will remove the data from Pool Members, filters, and automations.