Remove all NativeSwapChain*

These were helpers to avoid the need for users of implementation-based
swapchains to implement them themselves. They are no longer used and can
be removed.

Also removes SwapChainuUtils.h that's no longer used.

Bug: dawn:269
Change-Id: I3c563edaf107e9ce04f708e8bacae9d231b0f40d
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/126421
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Loko Kung <lokokung@google.com>

include/dawn/native/NullBackend.h

@@ -15,11 +15,12 @@
 #ifndef INCLUDE_DAWN_NATIVE_NULLBACKEND_H_
 #define INCLUDE_DAWN_NATIVE_NULLBACKEND_H_
 
-#include "dawn/dawn_wsi.h"
 #include "dawn/native/DawnNative.h"
 
 namespace dawn::native::null {
-DAWN_NATIVE_EXPORT DawnSwapChainImplementation CreateNativeSwapChainImpl();
+
+// Nothing for now \o/
+
 }  // namespace dawn::native::null
 
 #endif  // INCLUDE_DAWN_NATIVE_NULLBACKEND_H_