cleanup: Remove redundant plotting.py utility file

Remove unused utils/plotting.py file that duplicates functionality
already present in tools/visualization.py.

Analysis confirmed:
- No downstream dependencies or imports
- Functions (create_spatial_plot, create_cluster_plot, create_heatmap) are unused
- Functionality completely superseded by tools/visualization.py
- 164 lines of dead code from initial commit
- No test coverage or module exposure

This cleanup eliminates code duplication and potential confusion while
maintaining all existing functionality through the robust visualization
system in tools/visualization.py.

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: cafferychen777