Removed comment in AGG backend that is no longer applicable

[pgtgrly]

PR Summary

I was going through the agg renderer backend code. It seems like swig is no longer used by the agg backend to integrate it with Python (as per the changelog file). So I removed te comment as it was a bit misleading.

PR Checklist

• Has Pytest style unit tests
• Code is PEP 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

[jklymak]

thanks a lot @pgtgrly !