net: bcmgenet: remove unused function in bcmgenet.c

We get 1 warning when building kernel with W=1:
drivers/net/ethernet/broadcom/genet/bcmgenet.c:2763:5: warning: no previous prototype for 'bcmgenet_hfb_add_filter' [-Wmissing-prototypes]

In fact, this function is implemented in
drivers/net/ethernet/broadcom/genet/bcmgenet.c, but be called
by no one, thus can be removed.

So this patch removes the unused functions.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>