configs: imx8m{m, n}_venice: remove unneeded CONFIG_FEC_MXC_PHYADDR

configs: imx8m{m, n}_venice: remove unneeded CONFIG_FEC_MXC_PHYADDR

The IMX8M based Venice boards all have device-tree fec nodes that
use proper dt with a phy-handle pointing to a phy with reg assigned
to the proper phy address.

There is no need to keep using the CONFIG_FEC_MXC_PHYADDR hack when
a proper dt is used - remove it.

This was previously done in commit c5169b4f156a
("configs: imx8m{m, n}_venice: remove unneeded CONFIG_FEC_MXC_PHYADDR")
but got clobbered by commit 0d123e98b705
("Convert CONFIG_SYS_BARGSIZE to Kconfig")

Signed-off-by: Tim Harvey <tharvey@gateworks.com>