This reverts commit e6e99c54.

Reason for revert: Seems to be flaking on linux-clang-rel:
https://ci.chromium.org/ui/p/angle/builders/try/linux-clang-rel/22602/overview

Original change's description:
> Vulkan: Load custom Vk Loader.
>
> Uses a GN copy rule to duplicate the loader. Also updates volk to
> load using the new custom loader. Once both are in place we can
> go back and remove the copy.
>
> Bug: chromium:1219969
> Change-Id: I8c48d168a842539f7cdba1ebfdaf3b08c3e1990d
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2982499
> Commit-Queue: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Geoff Lang <geofflang@chromium.org>
> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>

Bug: chromium:1219969
Change-Id: Ib29014e728c1ce63c5e5bf7136ce19de7692964d
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2987874
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>