ixgbe: remove umem from adapter

As current implementation of netdev already contains and provides
umems for us, we no longer have the need to contain these
structures in ixgbe_adapter.

Refactor the code to operate on netdev-provided umems.

Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>