Revert "[utils] Adds Span::FillBytes as a workaround for ASAN timeouts."

This reverts commit d0dc9fd134a521b7e68605a7bc6769c273e3e51b.

Reason for revert: memset probably needs to be wrapped in unsafe.

Failure Link: https://ci.chromium.org/ui/p/dawn/builders/ci/dawn-win-x64-builder-asan/3517/overview

Original change's description:
> [utils] Adds Span::FillBytes as a workaround for ASAN timeouts.
>
> - In the bug below, we identified that as a part of spanification,
>   we updated some memset calls to std::ranges::fill. While this
>   was good for normal builds, this caused timeout issues in ASAN
>   builds because under ASAN std::ranges::fill obscures the loop
>   structure from the compiler's loop recognition. When dealing
>   with large buffers, this translates to billions of instrumented
>   stores causing timeouts.
> - This change introduces the Span::FillBytes helper that addresses
>   these timeouts by falling back to using memset when built for
>   ASAN.
>
> Bug: 545842615
> Change-Id: I431d16af99036cf8a2ba45ef173aa119e115804f
> Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/338335
> Commit-Queue: dan sinclair <dsinclair@chromium.org>
> Auto-Submit: Loko Kung <lokokung@google.com>
> Reviewed-by: dan sinclair <dsinclair@chromium.org>
> Reviewed-by: Antonio Maiorano <amaiorano@google.com>

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

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Bug: 545842615
Change-Id: I179f09ee41cd9d39ad1838653bc10a385f6f0be5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/338775
Reviewed-by: Shrek Shao <shrekshao@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Loko Kung <lokokung@google.com>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>