To remove a custom field from your form, you have to follow the simple steps below:
let's say you have a form with 3 fields (see image below)
First name
Last name
Email
Let's say you want to remove the custom field "Last name" from your form
So how to do it:
First : Click on the field "Last name" on your form
Once you clicked, the "Custom field" will appear on the left side, then select any other field like "first name". (Important Note: you need to select a different field from the one you want to remove)
3. Once you select "an other field", in the case above I selected "Firstname", a red button "Remove Custom Field" Appears, just click on it to remove the field "Last name" from your form
4. Click on "save form" button to save changes