net: phy: at803x: remove qca8081 1G fast retrain and slave seed config

The fast retrain and slave seed configs are only applicable when the 2.5G
ability is supported.

Signed-off-by: Luo Jie <quic_luoj@quicinc.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>