Consolidate `outcome_names()` and `.get_tune_outcome_names()`

[hfrick]

When reviewing #994 I noticed that we have outcome_names() with methods for different objects and also .get_tune_outcome_names() (as part of various .get_tune_*() functions).

We can consolidate .get_tune_outcome_names() with outcome_names.tune_results():

tune/R/outcome-names.R

Lines 53 to 63 in a8f0c05

	#' @export
	#' @rdname outcome_names
	outcome_names.tune_results <- function(x, ...) {
	att <- attributes(x)
	if (any(names(att) == "outcomes")) {
	res <- att$outcomes
	} else {
	res <- NA_character_
	}
	res
	}

tune/R/utils.R

Lines 208 to 218 in a8f0c05

	#' @export
	#' @rdname tune_accessor
	.get_tune_outcome_names <- function(x) {
	x <- attributes(x)
	if (any(names(x) == "outcomes")) {
	res <- x$outcomes
	} else {
	res <- character(0)
	}
	res
	}