Don't explode if removing an absent file fails.

We still got the desired result: the file is gone. This bug has existed in
OkHttp's DiskLruCache for a long time, but it was masked by the fact that
our cleanup callable was being run with a Future, and we never asked that
Future whether it succeeded or failed. When we changed to call execute(),
the problem became unmasked.
Merge pull request #650 from square/jwilson_0314_remove_absent

Don't explode if removing an absent file fails.