When you're removing the fields from the field set, make sure you're removing them from the field mapping (Step 3) too

If you remove the Account Number from the field set and the Account Number is mapped in Step 3 so error will be rendered after clicking the Submit button. That's a reason Account Number needs to remove from the mapping. 

Go to the App and click on the About Update Client Detail Tab open Step 3 and remove the Account Number mapping.