Get rid of the vagrant-berkshelf plugin

It has been abandoned by its creators

Vagrantfile

@@ -27,13 +27,6 @@ Vagrant.configure(2) do |config|
     override.ssh.port = 22
   end
 
-  if Vagrant.has_plugin?("vagrant-berkshelf")
-    config.berkshelf.enabled = true
-  else
-    puts "Install vagrant-berkshelf:\n  vagrant plugin install vagrant-berkshelf"
-    exit 1
-  end
-
   # Disable automatic box update checking. If you disable this, then
   # boxes will only be checked for updates when the user runs
   # `vagrant box outdated`. This is not recommended.