chore: rm unused objects in plot scripts

[maelle]

No description provided.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 72605c3 is merged into main:

• ✔️as_adjacency_matrix: 769ms -> 767ms [-1.12%, +0.67%]
• 🚀as_biadjacency_matrix: 751ms -> 743ms [-1.81%, -0.12%]
• ✔️as_data_frame_both: 1.47ms -> 1.46ms [-1.95%, +0.74%]
• ❗🐌as_long_data_frame: 3.88ms -> 3.92ms [+0.03%, +2.3%]
• ✔️es_attr_filter: 2.67ms -> 2.67ms [-1.34%, +1.14%]
• ✔️graph_from_adjacency_matrix: 124ms -> 125ms [-0.85%, +2.8%]
• ✔️graph_from_data_frame: 3.33ms -> 3.35ms [-0.17%, +1.86%]
• ✔️vs_attr_filter: 1.53ms -> 1.53ms [-2.07%, +2.13%]
• ✔️vs_by_name: 968µs -> 980µs [-0.68%, +3.08%]
  Further explanation regarding interpretation and methodology can be found in the documentation.