r8169: remove unused member features from struct

Member features of struct rtl8169_private isn't used any longer since
commit 6c6aa15f "r8169: improve interrupt handling", so remove it.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>