fs: mounts: Remove unused declaration mnt_cursor_del()

Commit 2eea9ce4310d ("mounts: keep list of mounts in an rbtree")
removed the implementation but leave declaration.

Signed-off-by: Yue Haibing <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

fs/mount.h

···
 	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);