Remove spurious print statement

When running nbconvert, I see this output:

```
[NbConvertApp] Using existing profile dir: '/tmp'
[NbConvertApp] Converting notebook nbgrader assign.ipynb to notebook
[NbConvertApp] Support files will be in nbgrader assign_files/
[NbConvertApp] Executing notebook with kernel: python3
error
status
status
status
```

The "error" and "status" messages come from a print statement that appears to be a debug statement that wasn't removed. I've removed it in this pull request .