Clean some compiler warnings

Author: mgreter

src/ast_expressions.hpp

@@ -39,7 +39,7 @@ namespace Sass {
     }
 
     // Convert to string (only for debugging)
-    sass::string toString() const
+    sass::string toString() const override final
     {
       return "&";
     }

src/ast_nodes.hpp

@@ -164,14 +164,11 @@ namespace Sass {
     virtual sass::string toString() const = 0;
 
     // Declare up-casting methods
-    DECLARE_ISA_CASTER(IfExpression);
-    DECLARE_ISA_CASTER(CssFnExpression);
     DECLARE_ISA_CASTER(UnaryOpExpression);
     DECLARE_ISA_CASTER(BinaryOpExpression);
     DECLARE_ISA_CASTER(InvocationExpression);
     DECLARE_ISA_CASTER(ParenthesizedExpression);
     DECLARE_ISA_CASTER(VariableExpression);
-    DECLARE_ISA_CASTER(FunctionExpression);
     DECLARE_ISA_CASTER(BooleanExpression);
     DECLARE_ISA_CASTER(StringExpression);
     DECLARE_ISA_CASTER(NumberExpression);