Quick Answers: Clean up settings sub-toggle

- Remove `kAssistantQuickAnswersEnabled` pref
- Clean up its ref in assistant and quick answers code
- Clean up its settings bindings and settings ui
- Clean up its policy bindings

Bug: b/159670857
Test: Run existing tests
Change-Id: I4cbd620c1aa80a34a81b9316abaf4e649fa6eeab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2582804
Commit-Queue: Yue Li <[email protected]>
Reviewed-by: Maksim Ivanov <[email protected]>
Reviewed-by: Sam McNally <[email protected]>
Reviewed-by: Xiyuan Xia <[email protected]>
Reviewed-by: Avi Drissman <[email protected]>
Reviewed-by: Kyle Horimoto <[email protected]>
Reviewed-by: Jeroen Dhollander <[email protected]>
Cr-Commit-Position: refs/heads/master@{#838169}
diff --git a/chrome/browser/prefs/browser_prefs.cc b/chrome/browser/prefs/browser_prefs.cc
index 516be64a..25803f34 100644
--- a/chrome/browser/prefs/browser_prefs.cc
+++ b/chrome/browser/prefs/browser_prefs.cc
@@ -522,6 +522,10 @@
     "ash.launcher.assistant_privacy_info_dismissed";
 #endif  // BUILDFLAG(IS_CHROMEOS_ASH)
 
+// Deprecated 12/2020
+const char kAssistantQuickAnswersEnabled[] =
+    "settings.voice_interaction.quick_answers.enabled";
+
 // Register local state used only for migration (clearing or moving to a new
 // key).
 void RegisterLocalStatePrefsForMigration(PrefRegistrySimple* registry) {
@@ -618,6 +622,8 @@
   registry->RegisterBooleanPref(kAssistantPrivacyInfoDismissedInLauncher,
                                 false);
 #endif  // BUILDFLAG(IS_CHROMEOS_ASH)
+
+  registry->RegisterBooleanPref(kAssistantQuickAnswersEnabled, true);
 }
 
 }  // namespace
@@ -1271,6 +1277,9 @@
   // Added 12/2020
   profile_prefs->ClearPref(prefs::kWebAppsUserDisplayModeCleanedUp);
 
+  // Added 12/2020
+  profile_prefs->ClearPref(kAssistantQuickAnswersEnabled);
+
   // Please don't delete the following line. It is used by PRESUBMIT.py.
   // END_MIGRATE_OBSOLETE_PROFILE_PREFS
 }