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 <stephen@networkplumber.org>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Jay Vosburgh <fubar@us.ibm.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>