Make which-support feature non-optional (#13125)

# Description
Removes the `which-support` cargo feature and makes all of its
feature-gated code enabled by default in all builds. I'm not sure why
this one command is gated behind a feature. It seems to be a relic of
older code where we had features for what seems like every command.

tests/shell/pipeline/commands/internal.rs

@@ -550,7 +550,6 @@ fn dynamic_closure_rest_args() {
     assert_eq!(actual.out, "1, 2, 3");
 }
 
-#[cfg(feature = "which-support")]
 #[test]
 fn argument_subexpression_reports_errors() {
     let actual = nu!("echo (ferris_is_not_here.exe)");