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

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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.

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