Bug 1765179: Remove dom/webgpu/ffi. r=jgilbert DONTBUILD

Remove the unused source directory `dom/webgpu/ffi`.

The `wgpu_ffi_generated.h` header actually #included by
`gfx/wgpu_bindings/wgpu.h` is generated by `gfx/wgpu_bindings/moz.build` at
build time and placed in the obj directory. The copy in the source directory is
ignored, and it's confusing to keep it around.

Differential Revision: https://phabricator.services.mozilla.com/D143931

.clang-format-ignore

@@ -34,7 +34,6 @@ layout/style/nsStyleStructList.h
 # Autogenerated file
 gfx/gl/GLConsts.h
 gfx/webrender_bindings/webrender_ffi_generated.h
-dom/webgpu/ffi/wgpu_ffi_generated.h
 intl/components/src/UnicodeScriptCodes.h
 intl/unicharutil/util/nsSpecialCasingData.cpp
 intl/unicharutil/util/nsUnicodePropertyData.cpp