e1000: remove e1000_clean_tx_irq call from e1000_netpoll

The call to e1000_clean_tx_irq in e1000_netpoll can race with the call
to e1000_clean_tx_irq in e1000_clean.  With a small bit of tweaking to
to netpoll_send_skb to simulate a system that was under extreme stress,
I was able to reproduce these concurrent calls.  This can result in
multiple frees to the skbs on the tx ring buffer.

Dropping this call from e1000_netpoll should be fine since we can rely
on the calls in e1000_clean to do what is needed since napi will poll
the hardware just after calling poll_controller.

Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>