Fix comment in WriteBatchWithIndex::NewIteratorWithBase (#11636)

Summary:
Remove obsolete comment.

Support for WriteBatchWithIndex::NewIteratorWithBase when overwrite_key=false is added in https://github.com/facebook/rocksdb/pull/8135, as you can clearly see in the HISTORY.md.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11636

Reviewed By: jowlyzhang

Differential Revision: D47722955

Pulled By: ajkr

fbshipit-source-id: 4fa44a309d9708e9f4a1530918a9aaf7114c9032

include/rocksdb/utilities/write_batch_with_index.h

@@ -189,9 +189,6 @@ class WriteBatchWithIndex : public WriteBatchBase {
   // Will create a new Iterator that will use WBWIIterator as a delta and
   // base_iterator as base.
   //
-  // This function is only supported if the WriteBatchWithIndex was
-  // constructed with overwrite_key=true.
-  //
   // The returned iterator should be deleted by the caller.
   // The base_iterator is now 'owned' by the returned iterator. Deleting the
   // returned iterator will also delete the base_iterator.