Remove support for marble

This CL removes the enable_cros_on_device_assistant GN flag.
It is no longer used (once crrev.com/i/3424659 lands).

Bug: b/171307059
Test: Compiled
Change-Id: Ideae2a69e6359a5f98b973b8e53e8e5f5890f429
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2554910
Auto-Submit: Jeroen Dhollander <jeroendh@chromium.org>
Commit-Queue: Xiaohui Chen <xiaohuic@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834394}

chromeos/assistant/assistant.gni

@@ -4,7 +4,6 @@ import("//build/config/chromeos/ui_mode.gni")
 declare_args() {
   # Enable assistant implementation based on libassistant.
   enable_cros_libassistant = is_chromeos_ash && is_chrome_branded
-  enable_cros_on_device_assistant = false
 
   # Enable a fake microphone, which can replay audio files as microphone input.
   # See chromeos/assistant/tools/send-audio.sh