btrfs/254: fix non-unique fsid issue

This test case needs two scratch fs. For debugging purposes, a fsid is
kept known. However, mkfs-btrfs has a stringent and inconsistent
approach, to check if that fsid is already present on any of the other
disks in the system. (This does not apply to the virtual devices in the
system).

To avoid running into the non-unique fsid found error, remove the known
fsid.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>