[Chromecast] Cleanup --enable-cma-media-pipeline flag.

This flag has been removed and is currently a no-op. No need
to pass it into the test.

Review-Url: https://codereview.chromium.org/2567243002
Cr-Commit-Position: refs/heads/master@{#438027}

chromecast/BUILD.gn

@@ -85,7 +85,6 @@ cast_test_group("cast_tests") {
       args = [
         "--no-sandbox",
         "--enable-local-file-accesses",
-        "--enable-cma-media-pipeline",
         "--ozone-platform=cast",
         "--test-launcher-jobs=1",
       ]