DBWriteImpl: remove redundant code (#4450)

Summary:
in `WriteThread::LaunchParallelMemTableWriters`, there is `  write_group->running.store(write_group->size);
`
https://github.com/facebook/rocksdb/blob/master/db/write_thread.cc#L510
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4450

Differential Revision: D10201900

Pulled By: yiwu-arbug

fbshipit-source-id: 96c8fbbba5aff7ba8a6ceb3117a2bd7cc9b2f34b

db/db_impl_write.cc

@@ -338,8 +338,6 @@ Status DBImpl::WriteImpl(const WriteOptions& write_options,
           }
         }
         write_group.last_sequence = last_sequence;
-        write_group.running.store(static_cast<uint32_t>(write_group.size),
-                                  std::memory_order_relaxed);
         write_thread_.LaunchParallelMemTableWriters(&write_group);
         in_parallel_group = true;