ARM: at91: remove ksz8081 phy fixup registration for sama5d4ek board

Commit 2b0ba96c ("net: phy: micrel: disable NAND-tree for KSZ8021,
KSZ8031, KSZ8051, KSZ8081") automated the NAND-tree mode deactivation
process, thus making this phy fixup useless.
Remove it along with the associated headers inclusion.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[nicolas.ferre@atmel.com: remove selection of PHYLIB in at91 Kconfig]
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>