Revert "rtnl: Add support for netdev event to link messages"

This reverts commit def12888.

As per discussion between Roopa Prabhu and David Ahern, it is
advisable that we instead have the code collect the setlink triggered
events into a bitmask emitted in the IFLA_EVENT netlink attribute.

Signed-off-by: David S. Miller <davem@davemloft.net>