Fix unused warning, remove Incr_batch_size dead code in sync

There is a Wunused-private-field warning:
```
In file included from /kvrocks/utils/kvrocks2redis/sync.cc:21:
/kvrocks/utils/kvrocks2redis/sync.h:61:5: warning: private field 'incr_state_' is not used [-Wunused-private-field]
  } incr_state_ = Incr_batch_size;
    ^
1 warning generated.
```

utils/kvrocks2redis/sync.h

@@ -54,12 +54,6 @@ class Sync {
   int next_seq_fd_;
   rocksdb::SequenceNumber next_seq_ = static_cast<rocksdb::SequenceNumber>(0);
 
-  // Internal states managed by IncrementBatchLoop procedure
-  enum IncrementBatchLoopState {
-    Incr_batch_size,
-    Incr_batch_data,
-  } incr_state_ = Incr_batch_size;
-
   Status incrementBatchLoop();
 
   Status tryCatchUpWithPrimary();