Remove some dead code about "acknowledged types"

components/sync_driver/pref_names.cc

Index: components/sync_driver/pref_names.cc
diff --git a/components/sync_driver/pref_names.cc b/components/sync_driver/pref_names.cc
index d4f04d3790b3757e2975b50378870db3b6f2158d..6ce65859c89aa39939112c5b4f69c14112fe285b 100644
--- a/components/sync_driver/pref_names.cc
+++ b/components/sync_driver/pref_names.cc
@@ -86,10 +86,6 @@ const char kSyncKeystoreEncryptionBootstrapToken[] =
 // passphrase.
 const char kSyncUsingSecondaryPassphrase[] = "sync.using_secondary_passphrase";
 
-// List of the currently acknowledged set of sync types, used to figure out
-// if a new sync type has rolled out so we can notify the user.
-const char kSyncAcknowledgedSyncTypes[] = "sync.acknowledged_types";
-
 // The GUID session sync will use to identify this client, even across sync
 // disable/enable events.
 const char kSyncSessionsGUID[] = "sync.session_sync_guid";