refactor(cubesql): Remove Extension from LogicalPlanLanguage::CubeScan

Extensions is used meaningfully only for CubeScan, but not for CubeScanWrapper. Both are first-class logical plan nodes for us, so Extension is not helpful in egraph

Author: mcheshkov

rust/cubesql/cubesql/src/compile/rewrite/analysis.rs

@@ -1242,7 +1242,6 @@ impl LogicalPlanAnalysis {
                 Some(c.to_string())
             }
             LogicalPlanLanguage::CubeScan(params) => cube_reference(params[0]),
-            LogicalPlanLanguage::Extension(params) => cube_reference(params[0]),
             _ => None,
         }
     }