btrfs/028: removed redundant sync and scratch filesystem unmount

btrfs/028: removed redundant sync and scratch filesystem unmount

There's no need to have an explicit scratch filesystem sync and unmount
at the of the test, as the fstests framework automatically unmounts the
filesystem and the unmount naturally syncs any data and metadata.

So remove them and update the comment to be more clear.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>