bonding: remove ifenslave.c from kernel source

bonding: remove ifenslave.c from kernel source

As Stephen proposed:
Since bonding supports configuration via iproute (netlink) and sysfs, I think
it is time to purge the old ifenslave code out of Documentation/networking
and update the documentation.

Suggested-by: Stephen Hemminger <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Cc: Jay Vosburgh <[email protected]>
Cc: "David S. Miller" <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Signed-off-by: David S. Miller <[email protected]>