Remove many unused exercise files

There are still a few unused ones waiting to be deployed (or waiting for me to double check w/ elizabeth about)

Test Plan:
Run this:
```
from contextlib import closing
import glob
import json
import urllib2

files = set([f.split('/')[1] for f in glob.glob("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
```
Verify it outputs `set([])`
Also, manually verify the alternate files required by `exercises/file_experiments.py` and `exercises/handler_raw.py` are still there.

Auditors: alex