Remove duplicate features_drush_help() switch case for drush:features-revert

I discovered that 7.x-1.x has a duplicate case in drush_features_help for drush:features-revert.

    case 'drush:features-revert':
      return dt("Revert a feature module on your site.");
    case 'drush:features-revert-all':
      return dt("Revert all enabled feature module on your site.");
    case 'drush:features-revert':
      return dt("Revert a feature module on your site.");

The third case there is a duplicate of the first and can be removed.

Attached patch removes those two lines.