Remove implicit cast operators on ResultValue<T>

As it turned out to not provide a complete solution to the C++-API-change issue on logical-change of the C-API, we simply remove those implicit cast operators. That is, accessing the result and the value need to be explicit.

tests/DeviceFunctions/DeviceFunctions.cpp

@@ -67,6 +67,8 @@ int main( int /*argc*/, char ** /*argv*/ )
 
     vk::DeviceMemory       deviceMemory       = device->allocateMemory( {} );
     vk::UniqueDeviceMemory uniqueDeviceMemory = vk::UniqueDeviceMemory( deviceMemory, *device );
+
+    vk::ResultValue<std::vector<vk::UniquePipeline>> pipelines = device->createGraphicsPipelinesUnique( nullptr, {} );
   }
   catch ( vk::SystemError const & err )
   {