This reverts commit d06feeac.

Reason for revert:
Earlier CL breaks pre-rotation:
https://chromium-review.googlesource.com/c/angle/angle/+/2598584

Original change's description:
> Vulkan: Generate xfb support code in SPIR-V for extension path
>
> The only piece of code that's needed to be generated for the extension
> path is the following, at the right spot (right before depth correction
> and pre-rotation):
>
>     ANGLEXfbPosition = gl_Position;
>
> The SPIR-V transformer already has gl_Position loaded for depth
> correction and pre-rotation, so this change simply adds an OpStore to
> ANGLEXfbPosition.
>
> As a result of this change, @@ XFB-OUT @@ is no longer emitted if
> the transform feedback extension is supported.  With the above code now
> placed correctly for geometry shaders, transform feedback tests for
> geometry shaders are enabled.
>
> Bug: angleproject:3606
> Change-Id: I13a7956ab62a1a6b4196ff999442b99b50226c0f
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2617659
> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
> Reviewed-by: Charlie Lao <cclao@google.com>
> Reviewed-by: Jamie Madill <jmadill@chromium.org>

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

Change-Id: I74fa9fafe3c922cdb7cd09ee6351534e38528da9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: angleproject:3606
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2634048Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Tim Van Patten <timvp@google.com>