net: remove CONFIG_NET_MULTI (again)

CONFIG_NET_MULTI has been removed long time ago by commit 795428fc
("net: remove CONFIG_NET_MULTI") but 4 boards were added later which wasn't
found.  The patch is removing this reference from 4 boards.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>