Remove unused 'completed' allocation status

I feel silly making a pull request for a one-line change...

See also: https://github.com/genome/genome/pull/8

```
$ psql -U genome -c "SELECT status, COUNT(allocation) FROM disk.allocation GROUP BY status"
  status  |  count
----------+---------
 invalid  |    2713
 purged   |  975666
 archived |  293582
 active   | 9689992
```