net: mdio: Remove support for building C45 muxed addresses

The old way of performing a C45 bus transfer created a special
register value and passed it to the MDIO bus driver, in the hope it
would see the MII_ADDR_C45 bit set, and perform a C45 transfer. Now
that there is a clear separation of C22 and C45, this scheme is no
longer used. Remove all the #defines and helpers, to prevent any code
being added which tries to use it.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>