removed obsolete concat;

--- a/src/Pure/ML-Systems/polyml.ML	Mon Aug 02 11:33:18 1999 +0200
+++ b/src/Pure/ML-Systems/polyml.ML	Mon Aug 02 15:39:04 1999 +0200
@@ -13,13 +13,12 @@
 
 
 structure String =
-  struct
+struct
   fun substring (s,i,j) = StringBuiltIns.substring (s, i+1, j)
                           handle Substring => raise Subscript
   fun isPrefix s1 s2 = (s1 = substring(s2,0, size s1))
                        handle Subscript => false
-  val concat = implode
-  end;
+end;
 
 
 (** ML system related **)