TestKit backend: remove severity fill from summary serialization (#1198)

The driver API never had this field, so the backend was faking it.

Said API has been deprecated in other drivers and was now removed in 6.x.
Therefore, the backend fill can also be removed.

See also: neo4j-drivers/testkit#673

Author: robsdedude

testkitbackend/totestkit.py

@@ -61,7 +61,6 @@ def serialize_notification(n: neo4j.SummaryNotification) -> dict:
             "description": n.description,
             "severityLevel": n.severity_level.name,
             "category": n.category.name,
-            "severity": n.raw_severity_level,
             "rawCategory": n.raw_category,
             "rawSeverityLevel": n.raw_severity_level,
         }