style: remove unused itertools import

Removed itertools import that was no longer needed after refactoring.

Detected by ruff linter: F401 'itertools' imported but unused

✅ All CI checks pass locally:
  - ty check: passed
  - black: passed
  - ruff: passed
  - pytest: 250 passed, 94.78% coverage

Author: Doug Borg

src/openapi_python_generator/language_converters/python/model_generator.py

@@ -1,4 +1,3 @@
-import itertools
 from typing import List, Optional, Union
 
 import click