vfs: remove some unused old mount api code

Remove reconfigure_single, mount_single, and compare_single now
that no users remain.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Link: https://lore.kernel.org/r/20250205213931.74614-5-sandeen@redhat.com


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