keep top-level "Isabelle" executable -- now an alias for "isabelle jedit";

--- a/Admin/makedist	Mon Sep 26 23:43:52 2011 +0200
+++ b/Admin/makedist	Mon Sep 26 23:51:59 2011 +0200
@@ -178,7 +178,6 @@
     echo
   } >ANNOUNCE
 else
-  rm Isabelle Isabelle.exe
   perl -pi -e "s,val is_official = false,val is_official = true,g" src/Pure/ROOT.ML
 fi