net: dsa: mv88e6xxx: cleanup after phylink_pcs conversion

Now that mv88e6xxx is completely converted to using phylink_pcs
support, we have no need for the serdes methods. Remove all this
infrastructure. Also remove the __maybe_unused from
mv88e6xxx_pcs_select().

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>