net: ag71xx: remove unnecessary legacy methods

ag71xx may have a PCS, but it does not appear to support configuration
of the PCS in the current code. The functions to get its state merely
report that the link is down, and the AN restart function is empty.

Since neither of these functions will be called unless phylink's legacy
flag is set, we can safely remove these functions and indicate this is
a modern driver.

Should PCS support be added later, it will need to be modelled using
the phylink_pcs support rather than operating as a legacy driver.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>