issue #536: stop defining explicit localhost in inventory.

This was needed at some point in the past, but the tests don't seem to
care about it any more. We'll fix any CI breakage by changing the tests,
since verifying implicit localhost behaviour is important.

tests/ansible/hosts/localhost.hosts

@@ -1,9 +1,8 @@
 # vim: syntax=dosini
 
-# This must be defined explicitly, otherwise _create_implicit_localhost()
+# issue #511, #536: we must not define an explicit localhost, as some
-# generates its own copy, which includes an ansible_python_interpreter that
+# transport_config/python_path.yml needs to test the implicit localhost
-# varies according to host machine.
+# behaviour.
-localhost
 
 # This is only used for manual testing.
 [localhost-x10]