deleting a commit does not clear the sanity_test cache

After I deleted a commit, I could still see the sanity test failure for it.

this fixed that situation:

```
$ cd var/db/freshports/cache/general
$ sudo rm sanity_test_failures.6ef681c77508923dfbc1ea458160d3885740ae0e.PageSize100.PageNum1.html
```

Easy enough to find the commit in question. This might require a new type of cache clearing, but we shall see.