This reverts commit 5cf7472d.

Reason for revert: causes timeouts, see anglebug.com/5470

Original change's description:
> Vulkan: Ignore glFlush to reduce vkQueueSubmits in Asphalt 9
>
> Multithreaded apps can use the following pattern:
>
>   glDrawElements()
>   glFenceSync()
>   glFlush()
>   glWaitSync()
>
> This currently results in a vkQueueSubmit for every glFlush() to ensure
> that the work has landed in the command queue in the correct order.
> However, ANGLE can instead avoid the vkQueueSubmit during the glFlush()
> in this situation by instead flushing the ContextVk's commands and
> ending the render pass to ensure the commands are submitted in the
> correct order to the renderer. This improves performance for Asphalt 9
> by reducing frame times from 150-200msec to 35-55msec.
>
> Specifically, ANGLE will call flushCommandsAndEndRenderPass() when
> there is a sync object pending a flush or if the ContextVk is currently
> shared.
>
> Additionally, on all devices except Qualcomm, ANGLE can ignore all other
> glFlush() calls entirely and return immediately. For Qualcomm devices,
> ANGLE is still required to perform a full flush (resulting in a
> vkQueueSubmit), since ignoring the glFlush() reduces the Manhattan 3.0
> offscreen score by ~3%.
>
> Bug: angleproject:5306
> Bug: angleproject:5425
> Change-Id: I9d747caf5bf306166be0fec630a78caf41208c27
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2552718
> Commit-Queue: Tim Van Patten <timvp@google.com>
> Reviewed-by: Charlie Lao <cclao@google.com>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>

TBR=timvp@google.com,jmadill@chromium.org,cclao@google.com

Change-Id: I9886bf901a835d408b6a4b8be7ea408fa2121be0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: angleproject:5306
Bug: angleproject:5425
Bug: angleproject:5470
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2595032Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>