Remove stale PLOT_REFACTOR_PLAN.md

The MatPlot/MatWrap/Visuals/Output removal this plan described is complete:
no MatPlot*, Visuals*, MatWrap*, mat_plot_2d/visuals_2d references or
mat_plot/wrap/visuals directories remain in autolens/. The plot modules are
already the function-based design the plan targeted, so the plan document is
obsolete.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RSRerFzzFttWNeZ5C7U4fx

Author: claude