mash: Change CastConfigDelegate to a mojoified CastConfigClient.

ash/common/system/tray/system_tray_delegate.cc

Index: ash/common/system/tray/system_tray_delegate.cc
diff --git a/ash/common/system/tray/system_tray_delegate.cc b/ash/common/system/tray/system_tray_delegate.cc
index a028b891433b361ff0ea58d8184281a56c5e7d2c..69ebf40e338dab0aa93b95198e41cacb1e9a189e 100644
--- a/ash/common/system/tray/system_tray_delegate.cc
+++ b/ash/common/system/tray/system_tray_delegate.cc
@@ -123,10 +123,6 @@ bool SystemTrayDelegate::GetBluetoothDiscovering() {
   return false;
 }
 
-CastConfigDelegate* SystemTrayDelegate::GetCastConfigDelegate() {
-  return nullptr;
-}
-
 NetworkingConfigDelegate* SystemTrayDelegate::GetNetworkingConfigDelegate()
     const {
   return nullptr;