net/eth.c: drop obsolete at91rm9200 support

All available at91rm9200 boards have migrated to ar920t/at91 and
therefore to CONFIG_NET_MULTI.
The obsolete at91rm9200_miiphy_initialize() was removed in "ARM: remove
obsolete at91rm9200".

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
1 file changed