generic/520: Remove sync in clean_dir

generic/520: Remove sync in clean_dir

When I test this case on xfs, it may fail as below:
--------------------------------------------
 === link SCRATCH_MNT/A/foo SCRATCH_MNT/bar  with fsync SCRATCH_MNT/A ===
+umount: /mnt/xfstests/scratch: target is busy.
+        (In some cases useful info about processes that use
+         the device is found by lsof(8) or fuser(1))
---------------------------------------------

It fails because somethings is still using the fs when we call sync and then
try to unmount it. We can simply remove sync as the unmount is supposed to
persist the file/directory removals.

Signed-off-by: Yang Xu <xuyang2018.jy@cn.fujitsu.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>