This reverts commit 552f5fcb.

Reason for revert: Causing flakiness in "confirm no-op" CQ step.
See issue for more details.

Bug: chromium:997016

Original change's description:
> Vulkan: Use VK repos' internal BUILD.gn files
> 
> This retires custom BUILD.gn files for the Vulkan Headers, Tools,
> Validation-Layers, and Loader repos. They now have integrated BUILD.gn
> files so switched ANGLE to use those by default.
> Also Validation Layer, Loader, and Tools repos no longer uses codegen
> so this speeds up the build overall.
> Switched over from old VK_LAYER_LUNARG_standard_validation meta-layer
> to VK_LAYER_KHRONOS_validation unified layer.
> 
> This changes includes a temporary workaround to the Vulkan-Loader repo
> to remove its Window's build dependence on WDK. There are plans to
> remove that depenedence in Loader master, but until then we can use
> our custom branch workaround.
> It also includes a custom branch for validation layers that includes
> some build fixes for ANGLE.
> Finally, the layers were crashing on Android when attempting to use
> DebugUtils extension so forcing DebugReport on Android for now.
> 
> Bug: angleproject:2449
> Bug: angleproject:3320
> Bug: angleproject:3852
> Change-Id: I48b36acefcc7f2bc930eb72d6fdbc87bca24f833
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1610438
> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Commit-Queue: Tobin Ehlis <tobine@google.com>

TBR=ynovikov@chromium.org,tobine@google.com,syoussefi@chromium.org,jmadill@chromium.org

Change-Id: Iec97ba8cb5c38fa56bfd4c0afb0f7af8d68513ff
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: angleproject:2449, angleproject:3320, angleproject:3852
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1767363Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>