Add a missing key to PhabricatorFile for destroying Files

Summary: See PHI176. Depends on D18733. We issue a query when deleting files that currently doesn't hit any keys.

Test Plan:
Ran `./bin/remove destroy --force --trace F56376` to get the query.

Ran ##SELECT * FROM `file` WHERE storageEngine = 'blob' AND storageHandle = '23366' LIMIT 1## before and after the change.

Before:

```
mysql> explain SELECT * FROM `file` WHERE storageEngine = 'blob' AND storageHandle = '23366' LIMIT 1;
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| 1 | SIMPLE | file | ALL | NULL | NULL | NULL | NULL | 33866 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
1 row in set (0.01 sec)
```

After:

```
mysql> explain SELECT * FROM `file` WHERE storageEngine = 'blob' AND storageHandle = '23366' LIMIT 1;
+----+-------------+-------+------+---------------+------------+---------+-------------+------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------------+---------+-------------+------+------------------------------------+
| 1 | SIMPLE | file | ref | key_engine | key_engine | 388 | const,const | 190 | Using index condition; Using where |
+----+-------------+-------+------+---------------+------------+---------+-------------+------+------------------------------------+
1 row in set (0.00 sec)
```

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18734