net: phy: remove unused fixup unregistering functions

net: phy: remove unused fixup unregistering functions

No user of PHY fixups unregisters these. IOW: The fixup unregistering
functions are unused and can be removed. Remove also documentation
for these functions. Whilst at it, remove also mentioning of
phy_register_fixup() from the Documentation, as this function has been
static since ea47e70e476f ("net: phy: remove fixup-related definitions
from phy.h which are not used outside phylib").

Fixup unregistering functions were added with f38e7a32ee4f
("phy: add phy fixup unregister functions") in 2016, and last user
was removed with 6782d06a47ad ("net: usb: lan78xx: Remove KSZ9031 PHY
fixup") in 2024.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://patch.msgid.link/ff8ac321-435c-48d0-b376-fbca80c0c22e@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>