igb: PCIe devices do not need to unset MANC_ARP_ENA

Users reported that ARP's were lost with igb. The problem
is fixed by not enabling this manageability configuration
bit.

None of the release_manageability code is actually needed as the
normal device reset during a shutdown returns everthing to
the right condition automatically.

Signed-off-by: David Graham <david.graham@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>