Update ExecInitPartitionInfo comment

Remove the words "if not already done."  This obsolete wording
corresponds to an early development version of what became edd44738.

Author: Etsuro Fujita
Reviewed-by: Amit Langote
Discussion: https://postgr.es/m/5ADF117B.5030606@lab.ntt.co.jp

src/backend/executor/execPartition.c

@@ -294,8 +294,7 @@ ExecFindPartition(ResultRelInfo *resultRelInfo, PartitionDispatch *pd,
 
 /*
  * ExecInitPartitionInfo
- *		Initialize ResultRelInfo and other information for a partition if not
- *		already done
+ *		Initialize ResultRelInfo and other information for a partition
  *
  * Returns the ResultRelInfo
  */