only have second prep() throw warning if training argument is specified

[EmilHvitfeldt]

Both of the following used to throw the warning on main which is too much

library(recipes)

rec <- recipe(~ ., data = mtcars) |>
  step_center(mpg, disp) |>
  prep()

tmp <- rec |> prep()
tmp <- rec |> prep(training = mtcars)
#> Warning in prep(rec, training = mtcars): 
#> ! The previous data will be used by `prep()`.
#> ℹ The data passed using `training` will be ignored.

[github-actions]

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