Minor comment update in execPartition.c

Hi,

Here is a comment for ExecInitPartitionInfo:

 296  * ExecInitPartitionInfo
 297  *      Initialize ResultRelInfo and other information for a
partition if not
 298  *      already done

I think we should remove the words "if not already done" from that
comment because 1) that function is called if the partition wasn't
already initialized and 2) that function assumes that.  Attached is a
small patch for removing the words.

Best regards,
Etsuro Fujita