When using the Form Builder, you can right-click a form element with your mouse if using a PC or Windows based laptop, and that will bring up a Delete / Undelete option.
Apple / Mac laptops and computers are a bit different, depending on what you are using. Rather than a right-click, by default they use a Control-click setup. This is achieved by holding the Control key while you click on an element. This will have the same effect as the right-click, bringing up the Delete / Undelete option.
You can adjust these settings to suit your preference at any time. To help understand these settings, and how you can change them to better suit your use, you can check out this Right-click on Mac article on the Apple support website.