fix: cleaning up code

[gord02]

Description

This PR cleans up the substrait-explain codebase.

[Copilot]

Pull request overview

This PR focuses on low-risk cleanup in substrait-explain’s textifier code by removing dead/commented-out code and refactoring some relation-construction logic for clarity without changing behavior.

Changes:

• Removed a large block of commented-out Schema textification code from src/textify/types.rs.
• Refactored src/textify/rels.rs to reorganize helpers/impl blocks and replace some manual loops with iterator-based constructions.
• Deleted the unused src/structural/mod.rs module implementation (no remaining references found in the crate sources).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/textify/types.rs	Removes commented-out legacy Schema textifier block near the end of the file.
src/textify/rels.rs	Reorganizes helper functions and Relation impls; simplifies several vector constructions.
src/structural/mod.rs	Removes an apparently unused structural-value module (no mod structural; / crate::structural references found).

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