Remove plugin and template_id from required fields in JSON schema

The plugin and template_id properties are not required all the time.

For instance, the resource jobs stored in the `resource-results` fields
do not contain these fields, as the `plugin` is assumed to be
`resource`, and resource jobs cannot be instanciated using templates, so
`template_id` cannot be used.

submission-schema/schema.json

@@ -217,9 +217,7 @@
                 "io_log",
                 "name",
                 "outcome",
-                "status",
-                "plugin",
-                "template_id"
+                "status"
             ],
             "title": "Result"
         },