Finish up plugin porting to global display

Also remove display = display which does nothing

lib/ansible/plugins/cache/base.py

@@ -31,6 +31,7 @@ except ImportError:
 
 class BaseCacheModule(with_metaclass(ABCMeta, object)):
 
+    # Backwards compat only.  Just import the global display instead
     _display = display
 
     @abstractmethod

lib/ansible/plugins/cache/jsonfile.py

@@ -37,7 +37,6 @@ from ansible.utils.unicode import to_bytes
 
 try:
     from __main__ import display
-    display = display
 except ImportError:
     from ansible.utils.display import Display
     display = Display()