Allow MSVC to build with C++20

The DXC bug preventing this has now been fixed.

Bug: dawn:1881
Change-Id: I6319b31d12872bab56426bcda4c3ab0bfd8c6542
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/148023
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: James Price <jrprice@google.com>
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/build_overrides/build.gni b/build_overrides/build.gni
index 3eb366e..4f5653c 100644
--- a/build_overrides/build.gni
+++ b/build_overrides/build.gni
@@ -28,11 +28,6 @@
 # Enables assertions on safety checks in libc++.
 enable_safe_libcxx = true
 
-# Use C++17 instead of C++20 in MSVC builds
-# TODO(crbug.com/dawn/1881): Fix upstream DXC build on MSVC with C++20 enabled,
-# then remove this flag.
-msvc_use_cxx17 = true
-
 # Detect whether we can use the hermetic XCode like in Chromium and do so if
 # possible.
 if (host_os == "mac" && use_system_xcode == "") {