Remove inappropriate comments (#6371)

Summary:
The comments are for iterators, not Cleanable.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6371

Test Plan: no need

Differential Revision: D19727527

Pulled By: cheng-chang

fbshipit-source-id: c74aeffa27ea0ce15a36ff6f9694826712cd1c70

include/rocksdb/cleanable.h

@@ -5,16 +5,6 @@
 // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
-//
-// An iterator yields a sequence of key/value pairs from a source.
-// The following class defines the interface.  Multiple implementations
-// are provided by this library.  In particular, iterators are provided
-// to access the contents of a Table or a DB.
-//
-// Multiple threads can invoke const methods on an Iterator without
-// external synchronization, but if any of the threads may call a
-// non-const method, all threads accessing the same Iterator must use
-// external synchronization.
 
 #pragma once