refactor: remove dead discover_functions implementations

[KRRT7]

Summary

• Replace PythonSupport.discover_functions body with raise NotImplementedError — this method was dead code since the routing in find_all_functions_in_file always sends Python files to the AST-based _find_all_functions_in_python_file path
• Replace JavaScriptSupport.discover_functions body with raise NotImplementedError — only reachable via _find_all_functions_via_language_support, which is gated off for Python; JS/TS optimization is not yet active through the CLI

Test plan

• Verify existing Python optimization paths still work (--file, --all, -m pytest)
• Run existing test suite to confirm no regressions