netdevsim: assume CONFIG_NET_DEVLINK is always enabled

Since commit f6b19b35 ("net: devlink: select NET_DEVLINK
from drivers") adds implicit select of NET_DEVLINK for
netdevsim, the code does not have to deal with the case
when CONFIG_NET_DEVLINK is not enabled. So remove the ifcase.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>