Revert "Fix location selection for pixel center polyfill."

This reverts commit 5f59eb343ae38fd1e8f3a9f6d4c6709b2051c407.

Reason for revert: Caused a clusterfuzz error, appears to have created a duplicate location 0.

Original change's description:
> Fix location selection for pixel center polyfill.
>
> When creating the pixel center polyfill we need to create a location in
> which to assign the value. This location must match between the vertex
> and fragment shaders.
>
> It's currently possible to setup the WGSL shader such that the current
> mechanism will not select the same location.
>
> Instead, pass the location to use for the polyfill into the SPIR-V
> writer. Dawn will use the entry point information and the locations used
> in both the vertex and fragment shaders to select the lowest unused
> location.
>
> Fixed: 517522769
> Change-Id: Ica81e95c9ab89587be6773b1950ee01f7661eee9
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/313217
> Commit-Queue: Corentin Wallez <cwallez@chromium.org>
> Reviewed-by: Corentin Wallez <cwallez@chromium.org>
> Auto-Submit: dan sinclair <dsinclair@chromium.org>
> Reviewed-by: James Price <jrprice@google.com>

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

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Change-Id: I911614567f6131380d677758a83fffcf8f65aa15
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/314336
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>