Chore: remove unused fields (#15764)

api/fields/external_dataset_fields.py

-from flask_restful import fields  # type: ignore
-
-from libs.helper import TimestampField
-
-external_knowledge_api_query_detail_fields = {
-    "id": fields.String,
-    "name": fields.String,
-    "setting": fields.String,
-    "created_by": fields.String,
-    "created_at": TimestampField,
-}

api/fields/segment_fields.py

     "stopped_at": TimestampField,
     "child_chunks": fields.List(fields.Nested(child_chunk_fields)),
 }
-
-segment_list_response = {
-    "data": fields.List(fields.Nested(segment_fields)),
-    "has_more": fields.Boolean,
-    "limit": fields.Integer,
-}