Remove obsolete mock (#2869)

This mock was a remenant of file reading tests that became obsolete with
the centralization of that code.

Signed-off-by: Trevor Bramble <tbramble@chef.io>

test/helper.rb

@@ -177,7 +177,6 @@ def md.directory?
       '/etc/postgresql/9.5/main' => mockfile.call('9.5.main'),
       '/var/lib/postgresql/9.5/main' => mockfile.call('var.9.5.main'),
       '/etc/hosts' => mockfile.call('hosts'),
-      '/etc/hosts_empty' => emptyfile.call,
       'C:\windows\system32\drivers\etc\hosts' => mockfile.call('hosts'),
       '/etc/fstab' => mockfile.call('fstab'),
       'fstab_no_home' => mockfile.call('fstab_no_home'),