chore: rm unused variable from idx.R

[maelle]

The comment said "for what purpose?", it seems the answer is "for no purpose" 😹

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 83f7222 is merged into main:

• ✔️as_adjacency_matrix: 947ms -> 947ms [-1.63%, +1.72%]
• ✔️as_biadjacency_matrix: 803ms -> 804ms [-0.7%, +1%]
• ✔️as_data_frame_both: 1.84ms -> 1.81ms [-4.71%, +1.08%]
• ✔️as_long_data_frame: 4.47ms -> 4.54ms [-2.85%, +5.97%]
• ✔️es_attr_filter: 2.88ms -> 2.85ms [-4.12%, +1.52%]
• ✔️graph_from_adjacency_matrix: 153ms -> 152ms [-3.09%, +1.55%]
• ✔️graph_from_data_frame: 3.58ms -> 3.56ms [-2.31%, +1.33%]
• ❗🐌vs_attr_filter: 1.62ms -> 1.67ms [+0.4%, +4.97%]
• ✔️vs_by_name: 1.08ms -> 1.1ms [-3.5%, +5.59%]
  Further explanation regarding interpretation and methodology can be found in the documentation.