Remove unnecessary smgrimmedsync() when creating unlogged table.

Remove unnecessary smgrimmedsync() when creating unlogged table.

This became safe after commit 4b4798e138. The smgrcreate() call will
now register the segment for syncing at the next checkpoint, so we
don't need to sync it here. If a checkpoint happens before the
creation is WAL-logged, the records will be replayed when starting
recovery from the checkpoint. If a checkpoint happens after the WAL
logging, the checkpoint will fsync() it.

In the passing, clarify a comment in smgrDoPendingSyncs().

Discussion: https://www.postgresql.org/message-id/6e5bbc08-cdfc-b2b3-9e23-1a914b9850a9%40iki.fi
Reviewed-by: Robert Haas