fix(LET-4053): clean up old mcp client code
Remove commented-out and unused files in functions/mcp_client:
- Deleted base_client.py (fully commented out)
- Deleted sse_client.py (fully commented out)
- Deleted stdio_client.py (fully commented out)
- Deleted empty __init__.py
Kept types.py and exceptions.py as they contain active code still being used.