Get rid of the ability to unregister preferences.

chrome/browser/prefs/pref_model_associator.cc

Index: chrome/browser/prefs/pref_model_associator.cc
diff --git a/chrome/browser/prefs/pref_model_associator.cc b/chrome/browser/prefs/pref_model_associator.cc
index 07bde77b68e2284c875ddd83f95c4c95b2d53fc7..6773c2c3eb98a06419b67fd99d79e57e77761edf 100644
--- a/chrome/browser/prefs/pref_model_associator.cc
+++ b/chrome/browser/prefs/pref_model_associator.cc
@@ -399,11 +399,6 @@ bool PrefModelAssociator::IsPrefRegistered(const char* name) {
   return registered_preferences_.count(name) > 0;
 }
 
-void PrefModelAssociator::UnregisterPref(const char* name) {
-  DCHECK(synced_preferences_.count(name) == 0);
-  registered_preferences_.erase(name);
-}
-
 void PrefModelAssociator::ProcessPrefChange(const std::string& name) {
   if (processing_syncer_changes_)
     return;  // These are changes originating from us, ignore.