Remove optimize_children and replace with map_children

[alamb]

The idea is that the following method can be removed and replaced with map_children introduced in #5630

pub fn optimize_children(
    optimizer: &impl PhysicalOptimizerRule,
    plan: Arc<dyn ExecutionPlan>,
    config: &ConfigOptions,
) -> Result<Arc<dyn ExecutionPlan>> {
    let children = plan
        .children()
        .iter()
        .map(|child| optimizer.optimize(Arc::clone(child), config))
        .collect::<Result<Vec<_>>>()?;

    if children.is_empty() {
        Ok(Arc::clone(&plan))
    } else {
        with_new_children_if_necessary(plan, children).map(Transformed::into)
    }
}

Originally posted by @mingmwang in #5630 (comment)

I think this would be a good first issue because it is mostly mechanical and would expose the person to datafusion's code, build and testing process