Remove duplicate entry from __init__.py (#897)

### Summary
This PR fixes a duplicated line in `__init__.py`

### Checks
- [x] I've added/updated the relevant documentation
- [x] I've run `make lint` and `make format`

Co-authored-by: easonsshi <[email protected]>

Author: Lightblues

src/agents/__init__.py

@@ -206,7 +206,6 @@ def enable_verbose_stdout_logging():
     "ToolCallItem",
     "ToolCallOutputItem",
     "ReasoningItem",
-    "ModelResponse",
     "ItemHelpers",
     "RunHooks",
     "AgentHooks",