vrf: remove slave queue and private slave struct

The private slave queue and slave struct haven't been used for anything
and aren't needed, this allows to reduce memory usage and simplify
enslave/release. We can use netdev_for_each_lower_dev() to free the vrf
ports when deleting a vrf device. Also if in the future a private struct
is needed for each slave, it can be implemented via lower devices'
private member (similar to how bonding does it).

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>