net: mvpp2: remove unused 'tx_skb' field of 'struct mvpp2_tx_queue'

This commit remove a field of 'struct mvpp2_tx_queue' that is not used
anywhere.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>