Remove unnecessary smgropen() calls
Now that RelationCreateStorage() returns the SmgrRelation (since
commit
5c1560606dc), use that.
Author: Japin Li
Discussion: https://www.postgresql.org/message-id/ME3P282MB316600FA62F6605477F26F6AB6742@ME3P282MB3166.AUSP282.PROD.OUTLOOK.COM