chore(query): remove unused clickhouse handler

[KKould]

I hereby agree to the terms of the CLA available at: https://docs.databend.com/dev/policies/cla/

Summary

This PR removes the unused ClickHouse compatibility handler from query startup, routing, session handling, and related configuration surfaces.

It also cleans up the remaining ClickHouse handler config/env/template entries in tests, deployment assets, helper scripts, and fixtures so the dead endpoint is fully removed instead of only being disabled in code.

Validation performed:

cargo fmt --all
cargo clippy -p databend-query --tests -- -D warnings
cargo clippy -p databend-binaries --bin databend-query -- -D warnings

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - This is dead code/config cleanup; I validated with fmt and clippy but did not add or run new functional test cases.

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[chatgpt-codex-connector]

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[KKould]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. What shall we delve into next?

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