[client] remove default confidence value of 15 for sightings (opencti #6835)

Author: JeremyCloarec

pycti/utils/opencti_stix2.py

@@ -1378,7 +1378,7 @@ def import_sighting(
             created=stix_sighting["created"] if "created" in stix_sighting else None,
             modified=stix_sighting["modified"] if "modified" in stix_sighting else None,
             confidence=(
-                stix_sighting["confidence"] if "confidence" in stix_sighting else 15
+                stix_sighting["confidence"] if "confidence" in stix_sighting else None
             ),
             createdBy=extras["created_by_id"] if "created_by_id" in extras else None,
             objectMarking=(