Removes the SQL Editor Pretty Explain feature — the Explain tab, the Run
EXPLAIN ANALYZE action + shortcut, and its dead plumbing. It's been
gated off behind the `DisablePrettyExplainOnSqlEditor` kill switch for
weeks with no usage or complaints.
`ExplainVisualizer` / `isExplainQuery` are kept — they're used
independently by Query Insights, Query Performance, and the EditorPanel
quick-runner. Manually-run `EXPLAIN` queries still render as raw rows in
the Results tab.
Typecheck, lint, and all affected unit tests pass.
Closes FE-3930
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
* **Changes**
* Removed the SQL editor’s EXPLAIN execution workflow, including its
toolbar action, keyboard shortcut, utility tab, and visual query-plan
display.
* Simplified query execution to focus on standard results and charts.
* Improved result clearing when switching databases and refined
execution error handling.
* Updated SQL editor state and tests to reflect the streamlined
experience.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
* wip: explain tab in results editor
* updated to add sql explain
* updated to default back to results
* updated explain function
* updated case with multiple statements
* updated to reset explain query results
* added tests for semi colon comments
* feature: add explain w/ AI on pretty-explain tab (#41588)
* wip: added explain with AI
* wip: updated header with new buttons
* updated prompt
* remove any types
* removed unused flag
* updated header
* formatted code
