Added DeleteFile and DeleteDirectory functions to FS:USER and the archives.

Tested using https://github.com/archshift/ctru-test.
Yeah, I removed them with the functions I was working on, but seemed to have kept them with `CreateDirectory`/`OpenDirectory`. The right thing to do would probably be just pass along `FileSys::Path`, and do the error checking in the backend, right? (this would be where a certain path type is not supported, no?) Furthermore, I think just an if-statement would be more readable for error checking.