chromeos: Clean up ash feature flag HideArcMediaNotifications

The flag has been on-by-default since at least June 2021. There are no
references to it in chromeos (e.g. Tast) or google3 (e.g. experiments).
Remove the flag and clean up the code.

Bug: none
Test: ash_unittests
Change-Id: I9389ab38755e2ff963506d545341468a83961498
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5278220
Reviewed-by: Alex Newcomer <newcomer@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1257672}

diff --git a/ash/constants/ash_features.cc b/ash/constants/ash_features.cc
index a0be895..a22f211 100644
--- a/ash/constants/ash_features.cc
+++ b/ash/constants/ash_features.cc

@@ -1416,14 +1416,6 @@
              "HiddenNetworkWarning",
              base::FEATURE_DISABLED_BY_DEFAULT);
 
-// Enables hiding of ARC media notifications. If this is enabled, all ARC
-// notifications that are of the media type will not be shown. This
-// is because they will be replaced by native media session notifications.
-// TODO(beccahughes): Remove after launch. (https://crbug.com/897836)
-BASE_FEATURE(kHideArcMediaNotifications,
-             "HideArcMediaNotifications",
-             base::FEATURE_ENABLED_BY_DEFAULT);
-
 // When enabled, shelf navigation controls and the overview tray item will be
 // removed from the shelf in tablet mode (unless otherwise specified by user
 // preferences, or policy). This feature also enables "contextual nudges" for
@@ -3567,10 +3559,6 @@
   return base::FeatureList::IsEnabled(kHibernate);
 }
 
-bool IsHideArcMediaNotificationsEnabled() {
-  return base::FeatureList::IsEnabled(kHideArcMediaNotifications);
-}
-
 bool IsHideShelfControlsInTabletModeEnabled() {
   return base::FeatureList::IsEnabled(kHideShelfControlsInTabletMode);
 }

diff --git a/ash/constants/ash_features.h b/ash/constants/ash_features.h
index a3f8df5..08e7066 100644
--- a/ash/constants/ash_features.h
+++ b/ash/constants/ash_features.h

@@ -457,8 +457,6 @@
 COMPONENT_EXPORT(ASH_CONSTANTS) BASE_DECLARE_FEATURE(kHibernate);
 COMPONENT_EXPORT(ASH_CONSTANTS) BASE_DECLARE_FEATURE(kHiddenNetworkWarning);
 COMPONENT_EXPORT(ASH_CONSTANTS)
-BASE_DECLARE_FEATURE(kHideArcMediaNotifications);
-COMPONENT_EXPORT(ASH_CONSTANTS)
 BASE_DECLARE_FEATURE(kHideShelfControlsInTabletMode);
 COMPONENT_EXPORT(ASH_CONSTANTS) BASE_DECLARE_FEATURE(kHindiInscriptLayout);
 COMPONENT_EXPORT(ASH_CONSTANTS)
@@ -1052,7 +1050,6 @@
 COMPONENT_EXPORT(ASH_CONSTANTS)
 bool AreAnyGlanceablesTimeManagementViewsEnabled();
 COMPONENT_EXPORT(ASH_CONSTANTS) bool IsHibernateEnabled();
-COMPONENT_EXPORT(ASH_CONSTANTS) bool IsHideArcMediaNotificationsEnabled();
 COMPONENT_EXPORT(ASH_CONSTANTS) bool IsHideShelfControlsInTabletModeEnabled();
 COMPONENT_EXPORT(ASH_CONSTANTS)
 bool IsHoldingSpaceCameraAppIntegrationEnabled();

diff --git a/ash/public/cpp/external_arc/message_center/arc_notification_manager.cc b/ash/public/cpp/external_arc/message_center/arc_notification_manager.cc
index 8cd672f..1ceb010 100644
--- a/ash/public/cpp/external_arc/message_center/arc_notification_manager.cc
+++ b/ash/public/cpp/external_arc/message_center/arc_notification_manager.cc

@@ -636,10 +636,9 @@
     return true;
   }
 
-  // Media Notifications may be ignored if we have the native views based media
-  // session notifications enabled.
-  if (data->is_media_notification &&
-      features::IsHideArcMediaNotificationsEnabled()) {
+  // Media Notifications are ignored because we show native views-based media
+  // session notifications instead.
+  if (data->is_media_notification) {
     return true;
   }