_Really_ don't report an error when cleaning already-deleted files.

My earlier #266 had a missing comma in the test that was giving false
results, so the code didn't actually work right.  Mea culpa.
Mark `cant_be_deleted?` as a private class method
Merge pull request #269 from randycoulman/really-clean-missing-files

Really don't report an error when cleaning already-deleted files