Remove unused exercise files

Test Plan:
Run this thing that compares the live exercises to what files exist:

```
from contextlib import closing
import glob
import json
import urllib2

files = set([f.split('/')[2] for f in glob.glob("khan-exercises/exercises/*.html")])
required = set()

with closing(urllib2.urlopen("https://www.khanacademy.org/api/v1/exercises")) as exercise_data:
    exercises = json.loads(exercise_data.read().decode('utf-8'))
    for exercise in exercises:
        if exercise['live'] and not exercise['uses_assessment_items'] and not exercise['deleted']:
            required.add(exercise['file_name'])

print required - files
```

and make sure it prints `set([])`

Auditors: peter