If you have an iPhone or iPad that you don’t use anymore and it’s still logged into iCloud, you can remove it from Find My iPhone. When you remove your iPhone or iPad from iCloud and turn off Find My iPhone it also deactivates Activation Lock at the same time.
The main reason you would want to remove Find My iPhone from your iCloud settings would be if you wanted to give your iPhone or iPad away. Another reason would be if you wanted to list it on ManualOnline. So before you sell your device, you need to wipe your iPhone or iPad and erase your content and settings (in Settings > General > Reset > Erase All Content and Settings). When you go to erase all your info from your iPhone or iPad, you will be asked to turn off Find My iPhone and Activation Lock before you can reset your iPhone or iPad.
Remove an iOS device or Mac from Find My iPhone by turning off Find My iPhone
On an iOS device: Go to Settings > iCloud, then tap to turn off Find My [device].