[vk] Use ash's definition of ENUMERATE_PORTABILITY_KHR.

Use the `ash` crate's definition of
`vk::InstanceCreateFlags::ENUMERATE_PORTABILITY_KHR` in
`wgpu_hal::vulkan::Instance::init`, rather than re-declaring it
locally ourselves.

wgpu-hal/src/vulkan/instance.rs

@@ -667,11 +667,8 @@ impl crate::Instance<super::Api> for super::Instance {
                 })
                 .collect::<Vec<_>>();
 
-            const VK_INSTANCE_CREATE_ENUMERATE_PORTABILITY_BIT_KHR: u32 = 0x00000001;
             let create_info = vk::InstanceCreateInfo::builder()
-                .flags(vk::InstanceCreateFlags::from_raw(
-                    VK_INSTANCE_CREATE_ENUMERATE_PORTABILITY_BIT_KHR,
-                ))
+                .flags(vk::InstanceCreateFlags::ENUMERATE_PORTABILITY_KHR)
                 .application_info(&app_info)
                 .enabled_layer_names(&str_pointers[..layers.len()])
                 .enabled_extension_names(&str_pointers[layers.len()..]);