refactor: Use String to replace Expression in DeletePlan

[Xuanwo]

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Summary

Related to #7593

/// # TODO
///
/// From @xuanwo
///
/// Ideally, we need to use `Scalar` in DeletePlan.selection. But we met a
/// cycle deps here. So we have to change `selection` in String first, and
/// change into `Scalar` when our `Planner` has been moved out.
///
/// At this stage, DeletePlan's selection expr will be parsed twice:
///
/// - Parsed during `bind` to get column index and projection index.
/// - Parsed during `execution` to get the correct columns
///
/// It's an ugly but necessary price to pay. Without this, we would sink in
/// hell forever.

With this PR, bind_delete will not have external deps anymore.

I know this is not the best code we can write. But there is no other choice...

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1 Ignored Deployment

Name	Status	Preview	Updated
databend	⬜️ Ignored (Inspect)		Sep 19, 2022 at 6:07AM (UTC)