chore: rm unused code from iterators.R

[maelle]

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[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if f8bde1d is merged into main:

• ✔️as_adjacency_matrix: 813ms -> 818ms [-0.8%, +2%]
• ✔️as_biadjacency_matrix: 812ms -> 819ms [-1.15%, +3%]
• ✔️as_data_frame_both: 1.54ms -> 1.57ms [-0.93%, +3.92%]
• ✔️as_long_data_frame: 4.06ms -> 4.01ms [-3.96%, +1.43%]
• ✔️es_attr_filter: 2.86ms -> 2.89ms [-2.03%, +4.13%]
• ✔️graph_from_adjacency_matrix: 157ms -> 156ms [-3.75%, +1.97%]
• ❗🐌graph_from_data_frame: 3.54ms -> 3.57ms [+0.09%, +1.89%]
• ❗🐌vs_attr_filter: 1.59ms -> 1.63ms [+0.13%, +5.47%]
• ✔️vs_by_name: 1.11ms -> 1.12ms [-3.81%, +5.68%]
  Further explanation regarding interpretation and methodology can be found in the documentation.