chore: rm unused variable

[maelle]

No description provided.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if a839b11 is merged into main:

• ✔️as_adjacency_matrix: 774ms -> 766ms [-2.52%, +0.58%]
• ✔️as_biadjacency_matrix: 753ms -> 751ms [-1.92%, +1.53%]
• ✔️as_data_frame_both: 1.52ms -> 1.52ms [-2.45%, +2.29%]
• ✔️as_long_data_frame: 3.95ms -> 3.92ms [-2.59%, +1.16%]
• ✔️es_attr_filter: 2.72ms -> 2.72ms [-3.04%, +3.01%]
• ✔️graph_from_adjacency_matrix: 120ms -> 120ms [-1.52%, +2.31%]
• ✔️graph_from_data_frame: 3.47ms -> 3.46ms [-3.31%, +2.77%]
• ✔️vs_attr_filter: 1.55ms -> 1.58ms [-5.19%, +9.44%]
• ✔️vs_by_name: 1.07ms -> 1.06ms [-4.91%, +3.19%]
  Further explanation regarding interpretation and methodology can be found in the documentation.