Remove unused flag enable_headless_music_mode

This flag hasn't been used in code in a while. Delete
some unused code.

Test: local build, no breaks.
Bug: none.
Change-Id: Id642f5c9424332395d5abc5d338bb8fe9fcb602e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1667607
Commit-Queue: Brett Kilty <brettk@google.com>
Auto-Submit: Brett Kilty <brettk@google.com>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Sergey Volk <servolk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682924}

chromecast/BUILD.gn

@@ -595,7 +595,6 @@ buildflag_header("chromecast_buildflags") {
     "ENABLE_CAST_WAYLAND_SERVER=$enable_cast_wayland_server",
     "ENABLE_CHROMECAST_EXTENSIONS=$enable_chromecast_extensions",
     "ENABLE_EXTERNAL_MOJO_SERVICES=$enable_external_mojo_services",
-    "ENABLE_HEADLESS_MUSIC_MODE=$enable_headless_music_mode",
     "ENABLE_PLAYREADY=$enable_playready",
     "ENABLE_VIDEO_CAPTURE_SERVICE=$enable_video_capture_service",
     "ENABLE_VOLUME_TABLES_ACCESS=$enable_volume_tables_access",

chromecast/chromecast.gni

@@ -49,9 +49,6 @@ declare_args() {
   # volume streams to a single one.
   is_single_volume = false
 
-  # Set to true to start music apps in headless mode.
-  enable_headless_music_mode = false
-
   # Set to true to keep Android activities alive when they are moved to the
   # background.
   enable_background_activities = false