[Sync] Remove the feature flag kSyncDeleteClearedPref

The feature flag has been introduced as kill switch for the change in
https://chromium-review.googlesource.com/c/chromium/src/+/1530906

The change stick and then the feature flag isn't needed any more.
More details are in the linked bug.

Change-Id: I35bd6ff3b221b28e1df14af6156741ff487fca5e
Bug: 943579
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730160
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Commit-Queue: Marc Treib <treib@chromium.org>
Auto-Submit: Mohamed Amir Yosef <mamir@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683160}

components/sync_preferences/pref_model_associator.cc

@@ -38,14 +38,6 @@ namespace sync_preferences {
 
 namespace {
 
-// Enables deleting a pref from Sync if the the user clears it on a client. If
-// this feature is disabled, clearing a pref will cause setting it to the
-// default value instead of deleting it from sync.
-// TODO(crbug.com/943579): This has been introduced in M75 as a safety
-// mechanism, should be removed in M78 if no issues are observed.
-const base::Feature kSyncDeleteClearedPref{"SyncDeleteClearedPrefs",
-                                           base::FEATURE_ENABLED_BY_DEFAULT};
-
 const sync_pb::PreferenceSpecifics& GetSpecifics(const syncer::SyncData& pref) {
   DCHECK(pref.GetDataType() == syncer::PREFERENCES ||
          pref.GetDataType() == syncer::PRIORITY_PREFERENCES);
@@ -526,8 +518,7 @@ void PrefModelAssociator::ProcessPrefChange(const std::string& name) {
       LOG(ERROR) << "Failed to update preference.";
       return;
     }
-    if (!base::FeatureList::IsEnabled(kSyncDeleteClearedPref) ||
-        pref_accessor_->GetPreferenceState(type_, name).persisted_value) {
+    if (pref_accessor_->GetPreferenceState(type_, name).persisted_value) {
       // If the pref was updated, update it.
       changes.push_back(syncer::SyncChange(
           FROM_HERE, syncer::SyncChange::ACTION_UPDATE, sync_data));