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

include/net/switchdev.h

@@ -326,10 +326,6 @@ int call_switchdev_blocking_notifiers(unsigned long val, struct net_device *dev,
 				      struct switchdev_notifier_info *info,
 				      struct netlink_ext_ack *extack);
 
-void switchdev_port_fwd_mark_set(struct net_device *dev,
-				 struct net_device *group_dev,
-				 bool joining);
-
 int switchdev_handle_fdb_event_to_device(struct net_device *dev, unsigned long event,
 		const struct switchdev_notifier_fdb_info *fdb_info,
 		bool (*check_cb)(const struct net_device *dev),