of: net: kill of_get_nvmem_mac_address()

We've switched all users to nvmem_get_mac_address(). Remove the now
dead code.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>