net: phylink: strip out pre-March 2020 legacy code

Strip out all the pre-March 2020 legacy code from phylink now that the
last user of it is gone.

Reviewed-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>