net: switchdev: Remove unused declaration switchdev_port_fwd_mark_set()

Commit 6bc506b4 ("bridge: switchdev: Add forward mark support for stacked devices")
removed the implementation but leave declaration.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20230808145955.2176-1-yuehaibing@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>