Remove redundant PNG files for mccall_model_with_separation

Removed static PNG files that are now dynamically generated using myst-nb glue:
- mccall_resw_alpha.png
- mccall_resw_beta.png
- mccall_resw_c.png

These figures are now generated from the exercise solution code and displayed via glue:figure directives, eliminating the need for static files and ensuring figures always match the code.

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

Co-Authored-By: Claude <[email protected]>

Author: jstac