Remove unused and not compiling DropDownDemo

For ease of use the simulator uses a globbing expression to get all
screens source files. This one was picked up as well and lead to a
compilation error.

Author: NeroBurner

src/CMakeLists.txt

@@ -626,7 +626,6 @@ set(INCLUDE_FILES
         displayapp/screens/InfiniPaint.h
         displayapp/screens/StopWatch.h
         displayapp/screens/Paddle.h
-        displayapp/screens/DropDownDemo.h
         displayapp/screens/BatteryIcon.h
         displayapp/screens/BleIcon.h
         displayapp/screens/NotificationIcon.h