net: remove bond_slave_has_mac_rcu()

[ Upstream commit 8b0fdcdc ]

No caller since v3.16.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: e74216b8 ("bonding: fix macvlan over alb bond support")
Signed-off-by: Sasha Levin <sashal@kernel.org>