conversation API backend update (#360)

### What problem does this PR solve?


Issue link:#345

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

api/db/services/document_service.py

@@ -15,7 +15,7 @@
 #
 from peewee import Expression
 
-from api.db import TenantPermission, FileType, TaskStatus
+from api.db import FileType, TaskStatus
 from api.db.db_models import DB, Knowledgebase, Tenant
 from api.db.db_models import Document
 from api.db.services.common_service import CommonService