Revert "Add dep on third_party/webgpu-headers"

This reverts commit b94ef82.

Reason for revert: Using upstream header is not going to happen anytime soon.

Original change's description:
> Add dep on third_party/webgpu-headers
>
> This way, we can eventually use the upstream header / .yml
>
> Fixed: dawn:2556
> Change-Id: I2e9138aa053117f2bf99c07c4f0a9a840629efe9
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/186660
> Reviewed-by: Kai Ninomiya <kainino@chromium.org>
> Commit-Queue: Austin Eng <enga@chromium.org>

# Not skipping CQ checks because original CL landed > 1 day ago.

Change-Id: I7e02ca20c7d3cf72cf8f3703a21ae12e216c86ae
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/221254
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Fr <beaufort.francois@gmail.com>

.gitmodules

@@ -132,9 +132,6 @@
 	path = third_party/dxheaders
 	url = https://chromium.googlesource.com/external/github.com/microsoft/DirectX-Headers
 	gclient-condition = host_os != "win"
-[submodule "third_party/webgpu-headers"]
-	path = third_party/webgpu-headers
-	url = https://chromium.googlesource.com/external/github.com/webgpu-native/webgpu-headers
 [submodule "third_party/khronos/OpenGL-Registry"]
 	path = third_party/khronos/OpenGL-Registry
 	url = https://chromium.googlesource.com/external/github.com/KhronosGroup/OpenGL-Registry

DEPS

@@ -353,10 +353,6 @@ deps = {
     'condition': 'host_os != "win"',
   },
 
-  'third_party/webgpu-headers': {
-    'url': '{chromium_git}/external/github.com/webgpu-native/webgpu-headers@8049c324dc7b3c09dc96ea04cb02860f272c8686',
-  },
-
   'third_party/khronos/OpenGL-Registry': {
     'url': '{chromium_git}/external/github.com/KhronosGroup/OpenGL-Registry@5bae8738b23d06968e7c3a41308568120943ae77',
   },