This change removes the @@ XFB-DECL @@ marker.  The ANGLEXfbPosition
output is unconditionally emitted in VS, TES and GS by the translator,
and is appropriately decorated or removed by the SPIR-V transformer.

Bug: angleproject:3606
Change-Id: Ia76224f5a6d147362eeb2d288f05e333aaf75481
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2617658
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>