Don't hide the const iterators from the base class

The const iterators can use the base class directly.

Author: Thomas Kiley

src/util/symbol_table.h

@@ -122,15 +122,8 @@ class symbol_tablet : public symbol_table_baset
 
   typedef symbolst::const_iterator const_iteratort;
 
-  virtual const_iteratort begin() const
-  {
-    return internal_symbols.begin();
-  }
-
-  virtual const_iteratort end() const
-  {
-    return internal_symbols.end();
-  }
+  using symbol_table_baset::begin;
+  using symbol_table_baset::end;
 
   /// Check that the symbol table is well-formed
   void validate(const validation_modet vm = validation_modet::INVARIANT) const;