[hotfix][checkpointing] Make CheckpointBarrierUnaligner#processEndOfP…

…artition always return false

For unaligned checkpoint, no buffer is stored in BufferStorage so CheckpointBarrierUnaligner#processEndOfPartition should always return false.

This closes apache#11351.

flink-streaming-java/src/main/java/org/apache/flink/streaming/runtime/io/CheckpointBarrierUnaligner.java

@@ -206,7 +206,6 @@ public boolean processEndOfPartition() throws Exception {
  new CheckpointException(CheckpointFailureReason.CHECKPOINT_DECLINED_INPUT_END_OF_STREAM));
  // no chance to complete this checkpoint
  releaseBlocksAndResetBarriers();
- return true;
  }
  return false;
  }