Revert "[Fuchsia] Enable R8 and R8G8 buffer formats"

This reverts commit e2838f16.

Reason for revert: Causing test fail on Fuchsia x64 CI bot (crbug.com/1141702).

Bug: 1141702

Original change's description:
> [Fuchsia] Enable R8 and R8G8 buffer formats
>
> Sysmem now supports R8 and R8G8 image formats, so it can be used to R8
> and R8G8 GpuMemoryBuffers
>
> Bug: 1141538
> Change-Id: I790c6886d67f49c09f2e2c4a93db7a27450aa30f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2491995
> Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
> Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
> Auto-Submit: Sergey Ulanov <sergeyu@chromium.org>
> Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#820012}

TBR=kmarshall@chromium.org,sergeyu@chromium.org

Change-Id: Ia0fdb609077aded3b85fd577bab0aa5df5cc4250
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1141538
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2492917Reviewed-by: David Dorwin <ddorwin@chromium.org>
Commit-Queue: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820089}

ui/ozone/platform/scenic/sysmem_buffer_collection.cc

@@ -28,12 +28,6 @@ VkFormat VkFormatForBufferFormat(gfx::BufferFormat buffer_format) {
     case gfx::BufferFormat::YUV_420_BIPLANAR:
       return VK_FORMAT_G8_B8R8_2PLANE_420_UNORM;
 
-    case gfx::BufferFormat::R_8:
-      return VK_FORMAT_R8_UNORM;
-
-    case gfx::BufferFormat::RG_88:
-      return VK_FORMAT_R8G8_UNORM;
-
     case gfx::BufferFormat::BGRA_8888:
     case gfx::BufferFormat::BGRX_8888:
       return VK_FORMAT_B8G8R8A8_UNORM;
@@ -55,8 +49,6 @@ bool SysmemBufferCollection::IsNativePixmapConfigSupported(
     gfx::BufferFormat format,
     gfx::BufferUsage usage) {
   bool format_supported = format == gfx::BufferFormat::YUV_420_BIPLANAR ||
-                          format == gfx::BufferFormat::R_8 ||
-                          format == gfx::BufferFormat::RG_88 ||
                           format == gfx::BufferFormat::RGBA_8888 ||
                           format == gfx::BufferFormat::RGBX_8888 ||
                           format == gfx::BufferFormat::BGRA_8888 ||