Deprecate Berkshelf metadata task

[athompson-r7]

We should deprecate the Berkshelf, :metadata task, it isn't called directly from builderator and I doubt anyone is using this anymore.

This task can be replaced with the following knife command. Removing this would also allow us to remove the chef gem dependency.

From the cookbook directory:

knife cookbook metadata from file ./metadata.rb

@rapid7/platform-delivery-austin @rapid7/platform-delivery-belfast @rapid7/platform-delivery-cambridge