common/board_r.c: drop legacy and unused bi_enetaddr

The bi_enetaddr field in struct bd_info is write-only; nothing ever
reads back the value.

Moreover, the value we write is more or less random, and certainly not
something one can rely on: If the board has a writable environment and
the mac address has been stored there, we fetch that value. But if the
board doesn't, this code runs before initr_net() -> eth_initialize(),
and thus before the code in eth-uclass which fetches MAC addresses
from eeprom, fuses or whatnot and populates the (run-time) environment
with those values.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Tom Rini <trini@konsulko.com>