igb: remove unneeded cleaned variable in clean_tx_irq path

The cleaned variable can be replaced by the count of packets cleaned during
the tx interrupt routine so it can be removed.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>