refactor(frontend): use purified definition for altering source

[BugenZhao]

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What's changed and what's your intention?

No user-facing changes.

• Remove unnecessary code after we have purification.
• Add more test cases for snapshot testing.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• My PR contains critical fixes that are necessary to be merged into the latest release.

Documentation

• My PR needs documentation updates.

Release note

[BugenZhao]

• feat(frontend): allow specifying (partial) schema when creating table with derived schema #20203
• refactor(frontend): use purified definition for altering source #20182 👈 (View in Graphite)
• feat(frontend): use purified definition for replacing table #20131
• feat(frontend): allow schema change & sink into a CTAS table #20174
• main

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