fix: remove duplicate warning message for unsupported Claude Code version

Fixes #359

The warning about unsupported Claude Code version was being output twice:
once via logger.warning() and again via print() to stderr. This caused
duplicate messages in the console.

Changes:
- Removed the redundant print() statement
- Kept logger.warning() as the single source of the warning
- Removed unused sys import (automatically cleaned by ruff)

🤖 Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

src/claude_agent_sdk/_internal/transport/subprocess_cli.py

@@ -6,7 +6,6 @@ import os
 import platform
 import re
 import shutil
-import sys
 import tempfile
 from collections.abc import AsyncIterable, AsyncIterator
 from contextlib import suppress
@@ -657,7 +656,6 @@ class SubprocessCLITransport(Transport):
                                 "Some features may not work correctly."
                             )
                             logger.warning(warning)
-                            print(warning, file=sys.stderr)
         except Exception:
             pass
         finally: