Detection of this chip version has been disabled for few kernel versions now.
Nobody complained, so remove support for this chip version.

v2:
- fix a typo: RTL_GIGA_MAC_VER_40 -> RTL_GIGA_MAC_VER_50
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>