This warning is new for iOS 9 since it supports resizable apps with multi-window support. Previously, apps would be auto-scaled to new devices which had different hardware display resolutions to those in the launch image (which is basically how iOS detects the supported interface resolutions). Now, apps which support multi-window must define the resolutions they support by including a launch image or storyboard for all device screen types.
Unless you are re-writing your app to support multi-window, you will be requiring 'Full screen'. This is a hint to iOS that you do not support multi-window, and basically makes iOS 9 work like previous versions in this regard.
So, if you are submitting an app update for iOS 9, and you do not support multi-window, all you have to do is to go to your Project Target settings, select General, and under 'Deployment Info' look for the tick box called 'Requires Full Screen'. Tick this and the warnings will disappear.