settings: Add async system salt retrieval logic in DeviceOAuth2TokenServiceFactory

chromeos/cryptohome/system_salt_getter.cc

Index: chromeos/cryptohome/system_salt_getter.cc
diff --git a/chromeos/cryptohome/system_salt_getter.cc b/chromeos/cryptohome/system_salt_getter.cc
index f299f0f4744f37de6342ce0fbcc179c4c2e00ef5..1fa25a325056662694e7e9deb23fdfb1910569dd 100644
--- a/chromeos/cryptohome/system_salt_getter.cc
+++ b/chromeos/cryptohome/system_salt_getter.cc
@@ -38,10 +38,6 @@ std::string SystemSaltGetter::GetSystemSaltSync() {
   return system_salt_;
 }
 
-std::string SystemSaltGetter::GetCachedSystemSalt() {
-  return system_salt_;
-}
-
 void SystemSaltGetter::GetSystemSaltInternal(
     const GetSystemSaltCallback& callback,
     bool service_is_available) {