XFSQA 073 is still not correctly cleaning up $tmp,

XFSQA 073 is still not correctly cleaning up $tmp,
this is because $tmp.source_dir is not an empty dir.

This change recursively deletes everything in $tmp.source_dir.
Not required for $tmp.loop as it is used as a loopback mountpoint.

the ignore external MKFS_OPTIONS change is no longer required,
as _scratch_mkfs_xfs now handles conflicting options.
Merge of master-melb:xfs-cmds:28863a by kenmcd.

  cleanup tmp and don't kill mkfs options