net: drop !NET_MULTI bcm570x/tigon3 drivers

These drivers have never been converted to NET_MULTI, and they are only
used by one board (BMW).  So drop the drivers until someone feels like
rewriting them for NET_MULTI support.

Rather than punting the BMW board completely, just disable net support
in its board config.  Seems to build fine without it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>