xfstests: btrfs: 020 is bit diverted from its objective

xfstests: btrfs: 020 is bit diverted from its objective

Not sure what this test case wanted to achieve by deleting the
source device before the replace.

As per the comments the objective of this test case seems to be

~~~~
btrfs device replace test on RO fs

Regression test for commit:
bbb651e Btrfs: don't allow the replace procedure on read only filesystems
~~~~~

Also there won't be EIO when you delete a loop device when its
still mounted. as shown below.

mount /dev/loop0 /mnt
losetup -d /dev/loop0
echo $?
0
dd if=/dev/zero of=/mnt/tf1 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.00192936 s, 265 kB/s
cd /mnt
sync
losetup -a
/dev/loop0: [0802]:1291816 (/root/testdev/disk1)

No errors in the dmesg as well.

Instead of further confusing, I am deleting  the delete loop device part

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>