`raise_if_unreachable` config flag appears to be unused

[phiggins]

There is a config flag raise_if_unreachable defined in the attributes file: https://github.com/chef-cookbooks/audit/blob/master/attributes/default.rb#L52

The only other reference to that string I can find is this instance variable reference: https://github.com/chef-cookbooks/audit/blob/master/libraries/helper.rb#L64

That instance variable is not set any where so that config option appears to not have any effect.