include: configs: rk3399: drop a dangling comment

Drop a irrelevent comment now the related configs have moved
to the various config files.
Signed-off-by: NPeter Robinson <pbrobinson@gmail.com>
Reviewed-by: NKever Yang <kever.yang@rock-chips.com>

include/configs/rk3399_common.h

@@ -75,6 +75,4 @@
 
 #endif
 
-/* enable usb config for usb ether */
-
 #endif