Revert "[gn] Add support for generated sources"

This reverts commit ad0da6af0764fbdfd6a673e3ce14c27ac948e6b0.

Reason for revert: break iOS builds in Chromium due to failure to find Go executable

Original change's description:
> [gn] Add support for generated sources
>
> Add a custom action that invokes the source generator. The build file
> generator produces a list of dependencies for this custom command, so
> that source generation will automatically re-run whenever any .tmpl
> file or .def file is changed.
>
> For GN we also pre-generate a list of generated sources because it
> needs to know what the outputs of the command are. Refactor some of
> the common code for generating these lists into helper functions.
>
> Bug: 513255285
> Change-Id: I8bd53a4073ecc099b89b5c7f584278936dcbf2f1
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/309180
> Reviewed-by: dan sinclair <dsinclair@chromium.org>
> Commit-Queue: James Price <jrprice@google.com>

TBR=dsinclair@chromium.org,jrprice@google.com,dawn-scoped@luci-project-accounts.iam.gserviceaccount.com

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 513255285
Change-Id: I59e7dd68df16740d82dbbc31d965b74491cf6856
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/310475
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: Alan Baker <alanbaker@google.com>
Commit-Queue: James Price <jrprice@google.com>