igb: don't read eicr when responding to legacy interrupts

The interrupt handler was reading eicr and then doing nothing with the
result.  I have removed the variable and the register read since they
provide no value to the legacy interrupt handler.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>