remove expected error string - current implementation emits compiler …

…specific error message e.g

```test/compilation_failure/is_type.cpp:8:7: error: no matching member function for call to 'is'
    x.is<std::string>();
    ~~^~~~~~~~~~~~~~~
./include/mapbox/variant.hpp:657:27: note: candidate template ignored: disabled by 'enable_if' [with T = std::basic_string<char>]
                          (detail::direct_type<T, Types...>::index != detail::invalid_value)>::type* = nullptr>
                          ^
./include/mapbox/variant.hpp:664:26: note: candidate template ignored: disabled by 'enable_if' [with T = std::basic_string<char>]
                         (detail::direct_type<recursive_wrapper<T>, Types...>::index != detail::invalid_value)>::type* = nullptr>
                         ^
1 error generated.
```
(ref #102)

test/compilation_failure/is_type.cpp

@@ -1,4 +1,4 @@
-// @EXPECTED: invalid type in T in `is<T>()` for this variant
+// @EXPECTED:
 
 #include <mapbox/variant.hpp>