net: remove netdevice gso_min_segs

After introduction of ndo_features_check(), we believe that very
specific checks for rare features should not be done in core
networking stack.

No driver uses gso_min_segs yet, so we revert this feature and save
few instructions per tx packet in fast path.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>