Remove unnecessary word in a comment

Relations opened by the executor are only closed once in
ExecCloseRangeTableRelations(), so the word "again" in the comment
for ExecGetRangeTableRelation() is misleading and unnecessary.

Discussion: https://postgr.es/m/CA+HiwqHnw-zR+u060i3jp4ky5UR0CjByRFQz50oZ05de7wUg=Q@mail.gmail.com
Backpatch-through: 12

src/backend/executor/execUtils.c

@@ -782,7 +782,7 @@ ExecInitRangeTable(EState *estate, List *rangeTable)
  * ExecGetRangeTableRelation
  *		Open the Relation for a range table entry, if not already done
  *
- * The Relations will be closed again in ExecEndPlan().
+ * The Relations will be closed in ExecEndPlan().
  */
 Relation
 ExecGetRangeTableRelation(EState *estate, Index rti)