Remove dawn::native::ExternalImageDescriptorIOSurface::plane

It is no longer used anywhere.

Bug: None
Change-Id: Ife3fa9470f3059efe34e29aa96adb2922ce91ebc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/129580
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
diff --git a/include/dawn/native/MetalBackend.h b/include/dawn/native/MetalBackend.h
index 4fd2752..a41cb6b 100644
--- a/include/dawn/native/MetalBackend.h
+++ b/include/dawn/native/MetalBackend.h
@@ -62,9 +62,6 @@
 
     IOSurfaceRef ioSurface;
 
-    // This has been deprecated.
-    uint32_t plane;
-
     // A list of events to wait on before accessing the texture.
     std::vector<ExternalImageMTLSharedEventDescriptor> waitEvents;
 };