remove unused member_exprt::symbol()

Reverts d47d503; the method isn't used.

Author: Daniel Kroening

src/util/std_expr.h

@@ -3918,18 +3918,6 @@ class member_exprt:public unary_exprt
   {
     return op0();
   }
-
-  // Retrieves the object(symbol) this member corresponds to
-  inline const symbol_exprt &symbol() const
-  {
-    const exprt &op=op0();
-    if(op.id()==ID_member)
-    {
-      return static_cast<const member_exprt &>(op).symbol();
-    }
-
-    return to_symbol_expr(op);
-  }
 };
 
 /*! \brief Cast a generic exprt to a \ref member_exprt