Several months ago I downloaded XCode 9.2 and I have my previous XCode 8.2.1 installed on my macbook air. However after I removed my previous XCode 8.2.1 in my Application folder and then installed the XCode 9.2, I still can use the XCode 8.2.1 to open the project. Now I have downloaded the XCode 9.4 in xip format. How can I completely uninstall the previous XCode versions (XCode 9.2 and XCode 8.2.1)?