net: phy: remove helper phy_is_internal

Helper phy_is_internal() is just used in two places phylib-internally.
So let's remove it from the API.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Mateusz Polchlopek <mateusz.polchlopek@intel.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/f3f35265-80a9-4ed7-ad78-ae22c21e288b@gmail.com


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