fix: remove spurious .flatten call that garbled SortMergeJoin fallback messages (#3968)

Fixes #3967. `flatMap` already returns a `Seq[String]`, so calling
`.flatten` on it triggered Scala's implicit String→Iterable[Char]
conversion, turning each message into individual characters.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: andygrove

spark/src/main/scala/org/apache/spark/sql/comet/operators.scala

@@ -2043,7 +2043,7 @@ object CometSortMergeJoinExec extends CometOperatorSerde[SortMergeJoinExec] {
     }
 
     if (errorMsgs.nonEmpty) {
-      withInfo(join, errorMsgs.flatten.mkString("\n"))
+      withInfo(join, errorMsgs.mkString("\n"))
       return None
     }