refactor: remove unused is_deleted field from conversations table

Remove the is_deleted field that was never utilized for soft deletion.
This simplifies queries and reduces unnecessary database overhead.

Fixes #25017

api/tests/test_containers_integration_tests/services/test_web_conversation_service.py

@@ -149,7 +149,6 @@ class TestWebConversationService:
             from_end_user_id=user.id if isinstance(user, EndUser) else None,
             from_account_id=user.id if isinstance(user, Account) else None,
             dialogue_count=0,
-            is_deleted=False,
         )
 
         from extensions.ext_database import db