fix: remove fake interactive implementations for line-interactive

matplotlib, seaborn, and plotnine cannot provide true interactivity
(hover, zoom, pan) when saving as static PNG. These implementations
used mplcursors and static annotations to "fake" interactivity, which
passed AI review but doesn't actually work in the saved output.

Removed:
- plots/line-interactive/implementations/matplotlib.py
- plots/line-interactive/implementations/seaborn.py
- plots/line-interactive/implementations/plotnine.py
- Corresponding metadata files
- GCS images

Remaining implementations (6): altair, bokeh, highcharts, letsplot, plotly, pygal

See #2842 for improvements to prevent this in future.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: MarkusNeusinger