net: phy: Drop phy_interface_is_sgmii

Recent commit 75d28899 ("net: phy: Synchronize PHY interface modes
with Linux") reordered the enum definitions. This caused the range of
enums that this api was checking to go bad.

There aren't anymore users of phy_interface_is_sgmii, so, just drop
it. Also the protocols are so very different that it makes no sense to
provide a helper wrapper in the hope of reuse for phy drivers.

Reported-by: Tom Rini <trini@konsulko.com>
Suggested-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Suggested-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/all/20230414103852.38705065@dellmb/


Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>