net: phylink: remove a bunch of unused validation methods

Remove exports for phylink_caps_to_linkmodes(),
phylink_get_capabilities(), phylink_validate_mask_caps() and
phylink_generic_validate(). Also, as phylink_generic_validate() is no
longer called, we can remove its implementation as well.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qsPkK-009wip-W9@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>