Revert "Reland: [gn] Add support for generated sources"

This reverts commit f4f0b9871ebdf71e06e0a51b5247828ca7ac36c0.

Reason for revert: broke the CrOS roll again :-(

Original change's description:
> Reland: [gn] Add support for generated sources
>
> Avoids use of absolute path to build directory when invoking the
> generation script, since this was causing failures on the bots that
> check for deterministic builds.
>
> Also ensures that the generate_sources action is only performed using
> the host toolchain, which fixes issues when cross-compiling.
>
> Original commit message below.
>
> 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.
>
> Like with Bazel, we also pre-generate a list of generated sources
> because it needs to know what the outputs of the command are.
>
> Bug: 513255285
> Change-Id: I08beed41653a7c40a6026bd3c690a0f4792c7f5b
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/311375
> Commit-Queue: James Price <jrprice@google.com>
> Reviewed-by: dan sinclair <dsinclair@chromium.org>

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

Bug: 513255285
Change-Id: Ib994188304857549b7124629323392b472dce193
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/312055
Commit-Queue: Brian Sheedy <bsheedy@google.com>
Reviewed-by: Michael Cui <mlcui@google.com>
Commit-Queue: James Price <jrprice@google.com>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Brian Sheedy <bsheedy@google.com>