Clean up old legacy preference migration logic.

components/user_prefs/tracked/pref_hash_store_impl.h

Index: components/user_prefs/tracked/pref_hash_store_impl.h
diff --git a/components/user_prefs/tracked/pref_hash_store_impl.h b/components/user_prefs/tracked/pref_hash_store_impl.h
index 254d8f3f89fbb0eb555ec65fe1f7f9a333bc5599..4a2520f0d891f71f63a029a372706205f1bc5ab1 100644
--- a/components/user_prefs/tracked/pref_hash_store_impl.h
+++ b/components/user_prefs/tracked/pref_hash_store_impl.h
@@ -41,11 +41,6 @@ class PrefHashStoreImpl : public PrefHashStore {
 
   ~PrefHashStoreImpl() override;
 
-  // Provides an external HashStoreContents implementation to be used.
-  // BeginTransaction() will ignore |storage| if this is provided.
-  void set_legacy_hash_store_contents(
-      std::unique_ptr<HashStoreContents> legacy_hash_store_contents);
-
   // Clears the contents of this PrefHashStore. |IsInitialized()| will return
   // false after this call.
   void Reset();
@@ -58,7 +53,6 @@ class PrefHashStoreImpl : public PrefHashStore {
   class PrefHashStoreTransactionImpl;
 
   const PrefHashCalculator pref_hash_calculator_;
-  std::unique_ptr<HashStoreContents> legacy_hash_store_contents_;
   bool use_super_mac_;
 
   DISALLOW_COPY_AND_ASSIGN(PrefHashStoreImpl);