Re-enable compiling with clang

Patch thanks to Gon Solo, issue 826

src/include/cppduals/duals/dual

@@ -485,9 +485,6 @@ struct is_arithmetic<duals::dual<T>> : is_arithmetic<T> {};
 
 #endif // CPPDUALS_ENABLE_IS_ARITHMETIC
 
-/// Duals are compound types.
-template <class T>
-struct is_compound<duals::dual<T>> : true_type {};
 
 // Modification of std::numeric_limits<> per
 // C++03 17.4.3.1/1, and C++11 18.3.2.3/1.