feat: remove enable_experimental_merge_into setting

[camilesing]

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

Summary

for issue: #18781

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - remove outdated setting

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 

[dantengsky]

https://github.com/databendlabs/databend/actions/runs/18525346931/job/52870788438?pr=18841#step:4:18198

java.sql.SQLException: Query failed，no need to retry: SQL: set enable_experimental_merge_into = 1 - Query failed: QueryErrors{code=2801, message=Unknown variable: "enable_experimental_merge_into"}
	at com.databend.jdbc.DatabendConnection.wrapException(DatabendConnection.java:830)
	at com.databend.jdbc.DatabendConnection.startQueryWithFailover(DatabendConnection.java:777)
	at com.databend.jdbc.DatabendConnection.startQuery(DatabendConnection.java:863)

@hantmac Do we need to keep backward compatible for setting enable_experimental_merge_into ?

[github-actions]

Docker Image for PR

• tag: pr-18841-b59676a-1760607312

note: this image tag is only available for internal use.

[github-actions]

ClickBench Report

• hits: https://benchmark.databend.com/clickbench/pr/18841/18555732111/hits.html
• tpch: https://benchmark.databend.com/clickbench/pr/18841/18555732111/tpch.html

[dantengsky]

LGTM, thanks for contributing!