This reverts commit 199a9f38.

Reason for revert: Causes a validation error when the extension is not present. See failures here: https://ci.chromium.org/p/chromium/builders/luci.chromium.try/win-angle-rel/693

Original change's description:
> Vulkan: support instanced draws.
> 
> Enable instanced draws with the Vulkan backend.
> So far it only works when Vulkan has VK_EXT_vertex_attribute_divisor.
> 
> BUG=angleproject:2672
> 
> Change-Id: I9445ba64282fa00a6eaee207b15efa2c7a9abbd3
> Reviewed-on: https://chromium-review.googlesource.com/c/1334973
> Commit-Queue: Frank Henigman <fjhenigman@chromium.org>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>

TBR=fjhenigman@chromium.org,jmadill@chromium.org,syoussefi@chromium.org

Change-Id: Iffccc2cca259bcd19c068a87a415d4e196901f45
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: angleproject:2672
Reviewed-on: https://chromium-review.googlesource.com/c/1468201Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>