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This reverts commit 6c92556c.

Reason for revert: Visual Studio 16.5.2 released, which fixes the issue,
and the workaround is (ironically) incompatible with 16.5.2.

Original change's description:
> build: workaround for VC v14.25 / MSVC 19.25
>
> Adds a temporary fix to vs_toolchain.py to use the previous version of
> MSVC's header files and documents how to install the previous version
> alongside 19.25.
>
> Bug: 1058860
> Change-Id: I6e96af3c71b23e5c5430237d00125a5830c37855
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2117732
> Commit-Queue: Richard Townsend <richard.townsend@arm.com>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Scott Violet <sky@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#753998}

TBR=sky@chromium.org,thakis@chromium.org,richard.townsend@arm.com

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

Bug: 1058860,1066935
Change-Id: I343aac7a0af7d89bf5497bf0d063e03731599f9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132379Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Richard Townsend <richard.townsend@arm.com>
Cr-Commit-Position: refs/heads/master@{#755490}