Revert "eclass/cros-rust: Fix CROS_WORKON_INCREMENTAL_BUILD."

This reverts commit feb938bd5352fb95c6ef83193bb518d359020684.

Reason for revert: Broke all the rust packages

Original change's description:
> eclass/cros-rust: Fix CROS_WORKON_INCREMENTAL_BUILD.
>
> Note this makes CROS_WORKON_INCREMENTAL_BUILD work for cros-rust ebuilds
> that have it set, but it doesn't address stale sources---changes to a
> crate in the cros_rust_registry that don't bump the version number will
> not be picked up by incremental builds yet.
>
> BUG=chromium:1165904
> TEST=emerge-${BOARD} p9 && emerge-${BOARD} p9
>
> Change-Id: I52e17a8c099b91905cdb7b72f214868d6eb6d168
> Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2625691
> Tested-by: Allen Webb <allenwebb@google.com>
> Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
> Commit-Queue: Allen Webb <allenwebb@google.com>

Bug: chromium:1165904
Change-Id: Iaabc8bb0e1fd2d26cfbb485ab4d3345eddf9c7c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/2627530
Reviewed-by: Fergus Dall <sidereal@google.com>
Reviewed-by: Allen Webb <allenwebb@google.com>
Commit-Queue: Fergus Dall <sidereal@google.com>
Tested-by: Fergus Dall <sidereal@google.com>
1 file changed