net: mv643xx_eth: Remove unused inline function sum16_as_be

commit 84411f73 ("net: mv643xx_eth: Avoid setting the initial TCP checksum")
left behind this, remove it.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>