Remove incorrect statistics warning

Summary:
statistics are always concurrently updated regardless of whether a single instance or multiple instances are used. remove the warning since it's unnecessarily preventing people from sharing stats objects across rocksdb instances.

replace #1819 which was having some problem with our internal tools.
Closes https://github.com/facebook/rocksdb/pull/1826

Differential Revision: D4495970

Pulled By: ajkr

fbshipit-source-id: 6601f61

include/rocksdb/options.h

@@ -946,8 +946,6 @@ struct DBOptions {
   uint64_t max_total_wal_size = 0;
 
   // If non-null, then we should collect metrics about database operations
-  // Statistics objects should not be shared between DB instances as
-  // it does not use any locks to prevent concurrent updates.
   std::shared_ptr<Statistics> statistics = nullptr;
 
   // If true, then the contents of manifest and data files are not synced