This reverts commit bedeb781.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Vulkan: Define VK_USE_PLATFORM_XLIB_KHR
> 
> For vulkan x11 build need to define VK_USE_PLATFORM_XLIB_KHR.
> 
> Bug: 1011068
> Change-Id: I1ff51a37618eb3a095bdf18e0615a3610225b6fc
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1863319
> Commit-Queue: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>

TBR=penghuang@chromium.org,tobine@google.com,jmadill@chromium.org

Change-Id: I6cf135e36603ea01bc418fd6f7ce7f131890531e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1011068
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1864579Reviewed-by: Tobin Ehlis <tobine@google.com>
Commit-Queue: Tobin Ehlis <tobine@google.com>