net: dsa: microchip: remove unused port_cleanup() callback

ksz_dev_ops :: port_cleanup() isn't used anywhere.

Remove it.

Signed-off-by: Bastien Curutchet (Schneider Electric) <bastien.curutchet@bootlin.com>
Link: https://patch.msgid.link/20260505-clean-ksz-driver-v1-2-05d70fa42461@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>