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