[chromecast] Remove --ozone-platform=cast and use headless everywhere.

healess is already added a few lines down as additional flags.

Bug: internal b/80500112
Test: None
Change-Id: Ibb57a383b1ceca74474258aecc7507d835ff3ff6
Reviewed-on: https://chromium-review.googlesource.com/1093118Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Commit-Queue: Gaofeng Huang <gfhuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565996}

chromecast/BUILD.gn

@@ -133,11 +133,6 @@ cast_test_group("cast_tests") {
         "--enable-local-file-accesses",
         "--test-launcher-jobs=1",
       ]
-      if (is_cast_audio_only) {
-        args += [ "--ozone-platform=headless" ]
-      } else {
-        args += [ "--ozone-platform=cast" ]
-      }
       if (!is_cast_desktop_build) {
         args += [ "--use-gpu-in-tests" ]