fstests: remove _wipe_fs from dump testing entirely

fstests: remove _wipe_fs from dump testing entirely

The _wipe_fs function in common/dump is a historical remnant of
xfstests, it's easy to cause confusion now. Now xfstests tend to
call `require_scratch && scratch_mkfs && scratch_mount` in each case
itself, we don't need to use a function to do that specially, so
remove _wipe_fs entirely.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>