Remove invalid check for broadcast partition id

Do not use the partition id assigned to the task for broadcast distributions.
Instead all tasks read from the same partition.

Fixes prestodb#4071

presto-main/src/main/java/com/facebook/presto/execution/scheduler/BroadcastOutputBufferManager.java

@@ -23,7 +23,6 @@
 
 import static com.facebook.presto.OutputBuffers.BROADCAST_PARTITION_ID;
 import static com.facebook.presto.OutputBuffers.INITIAL_EMPTY_OUTPUT_BUFFERS;
-import static com.google.common.base.Preconditions.checkArgument;
 import static java.util.Objects.requireNonNull;
 
 @ThreadSafe
@@ -51,8 +50,8 @@ public void addOutputBuffer(TaskId bufferId, int partition)
  return;
  }
 
- checkArgument(partition == BROADCAST_PARTITION_ID, "Broadcast partition must be %s", BROADCAST_PARTITION_ID);
- newOutputBuffers = outputBuffers.withBuffer(bufferId, partition);
+ // Note: it does not matter which partition id the task is using, in broadcast all tasks read from the same partition
+ newOutputBuffers = outputBuffers.withBuffer(bufferId, BROADCAST_PARTITION_ID);
  if (newOutputBuffers == outputBuffers) {
  return;
  }