Summary:
We has to remove this line because previously it is only called when use_os_buffer = false. But now we have direct io to replace it.
Closes https://github.com/facebook/rocksdb/pull/2573
Differential Revision: D5412824
Pulled By: yiwu-arbug
fbshipit-source-id: 81f3f0cdf94566bfc09ef2ff123e40cddbe36b36