Remove unstable public methods for DynamicFilterPhysicalExpr after proto migration #23422

pub struct Inner {
...
}

/// Return the filter's original children (before any remapping).
///
/// **Warning:** intended only for `datafusion-proto` (de)serialization.
/// Not a stable API.
pub fn original_children(&self) -> &[Arc<dyn PhysicalExpr>] {
    &self.children
}

/// Return the filter's remapped children, if any have been set via
/// [`PhysicalExpr::with_new_children`].
///
/// **Warning:** intended only for `datafusion-proto` (de)serialization.
/// Not a stable API.
pub fn remapped_children(&self) -> Option<&[Arc<dyn PhysicalExpr>]> {
    self.remapped_children.as_deref()
}