net: Kill link between CSUM and SG features.

Earlier SG was unset if CSUM was not available for given device to
force skb copy to avoid sending inconsistent csum.
Commit c9af6db4 (net: Fix possible wrong checksum generation)
added explicit flag to force copy to fix this issue.  Therefore
there is no need to link SG and CSUM, following patch kills this
link between there two features.

This patch is also required following patch in series.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>