`drop_na()` loses extra classes for `tbl_df`s

[d-morrison]

When I add a subclass to a tbl_df, drop_na() removes that class:

iris |> 
  dplyr::as_tibble() |> 
  structure(class = c("pop_data", "tbl_df", "tbl", "data.frame")) |>  
  tidyr::drop_na()  |>   
  class()
#> [1] "tbl_df"     "tbl"        "data.frame"

In contrast, it doesn't remove subclasses from data.frames:

iris |> 
  structure(class = c("pop_data", "data.frame")) |>  
  tidyr::drop_na() |>   
  class()
#> [1] "pop_data"   "data.frame"

^{Created on 2024-07-17 with reprex v2.1.0}

[DavisVaughan]

This is a specific case of #943