Don't store code tree structures in tests

[IndrajeetPatil]

Closes #1206

And also remove everything that was needed for R 3.2.

Script used to delete files:

tree_files_to_delete <-
  fs::dir_ls(
    path = ".",
    glob = "*.R|*.Rprofile|*.Rmd|*.Rmarkdown|*.Rnw|*.Qmd",
    invert = TRUE,
    type = "file",
    recurse = TRUE
  ) |>
  stringr::str_subset("_tree") |>
  purrr::map_chr(fs::path_abs)

stopifnot(all(fs::file_exists(tree_files_to_delete)))

fs::file_delete(tree_files_to_delete)

[IndrajeetPatil]

Please check that this is correct ignore directive.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 4f78252 is merged into main:

• ✔️cache_applying: 161ms -> 161ms [-2.78%, +2.16%]
• ✔️cache_recording: 526ms -> 527ms [-0.74%, +1.24%]
• ✔️without_cache: 987ms -> 988ms [-0.38%, +0.58%]

Further explanation regarding interpretation and methodology can be found in the documentation.