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.com


Signed-off-by: Christian Brauner <brauner@kernel.org>