Remove --diff mode support

In ansible 2.5 the way files are generated changes meaning that --diff
mode will stop working, and generate an error instead. Pending a better
solution, we should remove --diff mode.

Change-Id: I0d9b7baf08be00b978b8198f95db8962e7fa7787

action/config_template.py

@@ -672,18 +672,12 @@ class ActionModule(ActionBase):
         # Content from config_template is converted to src
         new_module_args.pop('content', None)
 
-        # Set the diff data before running the copy module
-        _diff = self._get_diff_data(_vars['dest'], transferred_data, task_vars)
-
         # Run the copy module
         rc = self._execute_module(
             module_name='copy',
             module_args=new_module_args,
             task_vars=task_vars
         )
-        if self._play_context.diff:
-            rc['diff'] = []
-            rc['diff'].append(_diff)
         if self._task.args.get('content'):
             os.remove(_vars['source'])
         return rc

tox.ini

@@ -34,7 +34,6 @@ setenv =
     ANSIBLE_LOOKUP_PLUGINS={toxinidir}/lookup
     ANSIBLE_STRATEGY_PLUGINS={toxinidir}/strategy
     ANSIBLE_LIBRARY={toxinidir}/library
-    ANSIBLE_PARAMETERS=--diff
 
 
 [testenv:docs]