dropped superfluous error message

--- a/src/HOL/Tools/SMT/smt_solver.ML	Mon Jan 10 17:37:11 2011 +0100
+++ b/src/HOL/Tools/SMT/smt_solver.ML	Mon Jan 10 17:39:54 2011 +0100
@@ -175,8 +175,6 @@
   val empty = Symtab.empty
   val extend = I
   fun merge data = Symtab.merge (K true) data
-    handle Symtab.DUP name => error ("Duplicate SMT solver: " ^ quote name)
-    (* FIXME never happens because of (K true) *)
 )
 
 local