Remove obsolete namespace mappings.

Summary:
The previous release 2.4 had a mapping to alias the older
namespace to rocksdb. This mapping is not needed in the new
release.

Test Plan:
make check
make release

Reviewers: emayanke

Reviewed By: emayanke

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13359

include/rocksdb/write_batch.h

@@ -96,6 +96,4 @@ class WriteBatch {
 
 }  // namespace rocksdb
 
-#include "rocksdb/rocksdb_to_leveldb.h"
-
 #endif  // STORAGE_ROCKSDB_INCLUDE_WRITE_BATCH_H_