[autotest] assistant_facade_native: Remove ChromeOSAssistant flag

Since ChromeOSAssistant flag is removed in CL:1767421, we remove the flag
usage in autotest.

BUG=chromium:946384, chromium:997307
TEST=Run audio_AudioBasicAssistant

Change-Id: I9674a20795de1935f1dca1f6bf82295b8e38587d
Reviewed-on: https://chromium-review.googlesource.com/1849591
Tested-by: Chih-Yang Hsia <paulhsia@chromium.org>
Commit-Ready: Chih-Yang Hsia <paulhsia@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Yu-Hsuan Hsu <yuhsuan@chromium.org>
diff --git a/client/cros/multimedia/assistant_facade_native.py b/client/cros/multimedia/assistant_facade_native.py
index d66ddf2..d02c89f 100644
--- a/client/cros/multimedia/assistant_facade_native.py
+++ b/client/cros/multimedia/assistant_facade_native.py
@@ -44,11 +44,7 @@
 
         if enable_dsp_hotword:
             custom_chrome_setup["extra_browser_args"] = (
-                ["--enable-features=ChromeOSAssistant,EnableDspHotword"])
-        else:
-            custom_chrome_setup["extra_browser_args"] = (
-                ["--enable-features=ChromeOSAssistant"])
-
+                ["--enable-features=EnableDspHotword"])
         self._resource.start_custom_chrome(custom_chrome_setup)