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.

crates/nu-cmd-lang/src/core_commands/version.rs

@@ -160,11 +160,6 @@ fn features_enabled() -> Vec<String> {
 
     // NOTE: There should be another way to know features on.
 
-    #[cfg(feature = "which-support")]
-    {
-        names.push("which".to_string());
-    }
-
     #[cfg(feature = "trash-support")]
     {
         names.push("trash".to_string());