Use the rm_r helper as much as possible

With rubyspec/mspec#29, the `rm_r` can be used to remove any kind of file without worrying, and `File.{unlink,delete}` is no more needed directly, especially in the form of `File.unlink file if File.exist? file` (which does not work for dangling symlinks). `rm_r` also prevents from removing file outside of `SPEC_TEMP_DIR`.

So I search rubyspec and replaced where it made sense. There are still some places where `File.{unlink,delete}` is left, because relative paths are used and they need bigger refactorings or because the spec wants to ensure the existence of the file or finally because some methods do not have the helpers loaded (for daemon specs).