r8169: remove unneeded dirty_rx index

After commit 6f0333b8 ("r8169: use 50% less ram for RX ring") the rx
ring buffers are always copied making dirty_rx useless.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Acked-by: Francois Romieu <romieu@fr.zoreil.com>
Signed-off-by: David S. Miller <davem@davemloft.net>