kitchen: drop stale chef_version pin

dokken defaults chef_version to "latest", so the 18.2.7 pin was both stale
and unnecessary. Removing it lets the suite track the current stable Cinc
Client.

kitchen.yml

@@ -1,7 +1,6 @@
 ---
 driver:
   name: dokken
-  chef_version: 18.2.7
   pull_platform_image: false
   pull_chef_image: false
   # memory_limit: 4294967296 # 4GB