fs: mounts: Remove unused declaration mnt_cursor_del()

Commit 2eea9ce4 ("mounts: keep list of mounts in an rbtree")
removed the implementation but leave declaration.
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20240803115000.589872-1-yuehaibing@huawei.comSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/mount.h

@@ -153,5 +153,4 @@ static inline void move_from_ns(struct mount *mnt, struct list_head *dt_list)
 	list_add_tail(&mnt->mnt_list, dt_list);
 }
 
-extern void mnt_cursor_del(struct mnt_namespace *ns, struct mount *cursor);
 bool has_locked_children(struct mount *mnt, struct dentry *dentry);