ixgbe: r_idx not used in ixgbe_msix_clean_rx()

The values of r_idx and rx_ring are not used after the last time they
are set in ixgbe_msix_clean_rx(), so they can be removed.

Signed-off-by: Ali Gholami Rudi <ali@rudi.ir>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>