[SQL][MINOR] Update comments and remove dead code in UnionLoop

### What changes were proposed in this pull request?

Remove function assertNoRecursiveCTE which is no longer used (when recursive references from subqueries were enabled, this function became obsolete).
Also edit comments and error message to reflect current state.

### Why are the changes needed?

Code cleanliness.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Existing golden file tests in cte-recursion and postgreSQL/with.sql.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #51097 from Pajaraja/pavle-martinovic_data/rctecleanup.

Authored-by: pavle-martinovic_data <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: Pajaraja

common/utils/src/main/resources/error/error-conditions.json

@@ -3416,14 +3416,9 @@
       "Invalid recursive reference found inside WITH RECURSIVE clause."
     ],
     "subClass" : {
-      "NUMBER" : {
-        "message" : [
-          "Multiple self-references to one recursive CTE are not allowed."
-        ]
-      },
       "PLACE" : {
         "message" : [
-          "Recursive references cannot be used on the right side of left outer/semi/anti joins, on the left side of right outer joins, in full outer joins, in aggregates, and in subquery expressions."
+          "Recursive references cannot be used on the right side of left outer/semi/anti joins, on the left side of right outer joins, in full outer joins and in aggregates"
         ]
       }
     },

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/CheckAnalysis.scala

@@ -947,30 +947,14 @@ trait CheckAnalysis extends LookupCatalog with QueryErrorsBase with PlanToString
     if (expr.plan.isStreaming) {
       plan.failAnalysis("INVALID_SUBQUERY_EXPRESSION.STREAMING_QUERY", Map.empty)
     }
-    assertNoRecursiveCTE(expr.plan)
     checkAnalysis0(expr.plan)
     ValidateSubqueryExpression(plan, expr)
   }
 
-  private def assertNoRecursiveCTE(plan: LogicalPlan): Unit = {
-    plan.foreach {
-      case r: CTERelationRef if r.recursive =>
-        throw new AnalysisException(
-          errorClass = "INVALID_RECURSIVE_REFERENCE.PLACE",
-          messageParameters = Map.empty)
-      case p => p.expressions.filter(_.containsPattern(PLAN_EXPRESSION)).foreach {
-        expr => expr.foreach {
-          case s: SubqueryExpression => assertNoRecursiveCTE(s.plan)
-          case _ =>
-        }
-      }
-    }
-  }
-
   /**
    * Validate that collected metrics names are unique. The same name cannot be used for metrics
-   * with different results. However multiple instances of metrics with with same result and name
-   * are allowed (e.g. self-joins).
+   * with different results. However, multiple instances of metrics with same result and name are
+   * allowed (e.g. self-joins).
    */
   private def checkCollectedMetrics(plan: LogicalPlan): Unit = {
     val metricsMap = mutable.Map.empty[String, CollectMetrics]

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -1041,8 +1041,7 @@ object ColumnPruning extends Rule[LogicalPlan] {
         p
       }
 
-    // TODO: Pruning `UnionLoop`s needs to take into account both the outer `Project` and the inner
-    //  `UnionLoopRef` nodes.
+    // Avoid pruning UnionLoop because of its recursive nature.
     case p @ Project(_, _: UnionLoop) => p
 
     // Prune unnecessary window expressions