Cleanup TextureCubeTestES3.CubeMapPixelUnpackBuffer Clean up TextureCubeTestES3.CubeMapPixelUnpackBuffer by removing the unused shader variable 'uLod'. Bug: b/194140619 Test: TextureCubeTestES3.CubeMapPixelUnpackBuffer Change-Id: I14776b34c5921fc4d4177bdaed205843b27b9e94 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3359777 Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Tim Van Patten <timvp@google.com>