remove Berksfile and Berksfile.lock from generated chefignore file

We recommend stripping the Berksfile or Berksfile.lock from cookbooks uploaded to the Chef Server - this is wrong. These files need to be redistributed to cookbook authors and available for tools who consume them.

Currently cookbooks retrieved via a Chef Server endpoint of a Berkshelf-API server will not contain the expected Berksfile and, if an Environment Cookbook, the Berksfile.lock.