avoid duplicate stderr output in isabelle build -v, e.g. "Timing ..." (amending a829207b32a3);

--- a/src/Pure/Tools/build.scala	Fri Mar 20 11:35:33 2020 +0100
+++ b/src/Pure/Tools/build.scala	Fri Mar 20 16:21:58 2020 +0100
@@ -294,7 +294,6 @@
             }
 
           process.result(
-            progress_stderr = Output.writeln(_),
             progress_stdout = (line: String) =>
               Library.try_unprefix("\floading_theory = ", line) match {
                 case Some(theory) =>