Can I delete unnecessary device simulators of Xcode?
Yes, you can delete any simulator that you don't use.  I do this routinely when I stop supporting older iOS versions.
If you delete them and then you find that you need them at some point in the future, you can redownload them from Apple's developer site.
The best way to delete them is in Xcode.  Go to Window -&gt; Devices and Simulators.  This will open a new window with all the devices you use in Xcode.
At the top, tap on Simulators and you'll see a list on the left-side.
From there, find the simulator you want to delete and Cntl - click (or right-click) and select Delete.
I do this with each simulator that runs in each iOS version that I no longer support.
Update July 2020: There's a free utility in the Mac App Store named DevCleaner for Xcode.  This application can display and delete simulators and various caches.  I've found it be a very quick and easy way to regain space.  I'm not the developer or associated with this application in any way.