Remove deprecated ExternalImageAccessDescriptorDXGIKeyedMutex behavior

Previously Dawn was using acquireKey + 1 for the releaseKey but we
changed the code transition to requiring a specific key. We supported
both options for a time but now that Chromium uses a releaseKey, we can
remove the deprecated option.

Bug: chromium:1213977
Change-Id: Iab253673c15b83b107dcc3635ee165cfa09efeb6
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/58383
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>

src/include/dawn_native/D3D12Backend.h

diff --git a/src/include/dawn_native/D3D12Backend.h b/src/include/dawn_native/D3D12Backend.h
index e545ee1..cfddcc2 100644
--- a/src/include/dawn_native/D3D12Backend.h
+++ b/src/include/dawn_native/D3D12Backend.h
@@ -56,8 +56,7 @@
         : ExternalImageAccessDescriptor {
       public:
         uint64_t acquireMutexKey;
-        // Release key will be set to acquireMutexKey + 1 if set to sentinel value UINT64_MAX.
-        uint64_t releaseMutexKey = UINT64_MAX;
+        uint64_t releaseMutexKey;
         bool isSwapChainTexture = false;
     };