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-command/src/default_context.rs

@@ -127,7 +127,7 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
             SysTemp,
             SysUsers,
             UName,
-
+            Which,
         };
 
         // Help
@@ -172,9 +172,6 @@ pub fn add_shell_command_context(mut engine_state: EngineState) -> EngineState {
         ))]
         bind_command! { Ps };
 
-        #[cfg(feature = "which-support")]
-        bind_command! { Which };
-
         // Strings
         bind_command! {
             Char,

crates/nu-command/src/system/which_.rs

@@ -92,7 +92,6 @@ fn get_entries_in_nu(
     all_entries
 }
 
-#[cfg(feature = "which-support")]
 fn get_first_entry_in_path(
     item: &str,
     span: Span,
@@ -104,17 +103,6 @@ fn get_first_entry_in_path(
         .ok()
 }
 
-#[cfg(not(feature = "which-support"))]
-fn get_first_entry_in_path(
-    _item: &str,
-    _span: Span,
-    _cwd: impl AsRef<Path>,
-    _paths: impl AsRef<OsStr>,
-) -> Option<Value> {
-    None
-}
-
-#[cfg(feature = "which-support")]
 fn get_all_entries_in_path(
     item: &str,
     span: Span,
@@ -129,16 +117,6 @@ fn get_all_entries_in_path(
         .unwrap_or_default()
 }
 
-#[cfg(not(feature = "which-support"))]
-fn get_all_entries_in_path(
-    _item: &str,
-    _span: Span,
-    _cwd: impl AsRef<Path>,
-    _paths: impl AsRef<OsStr>,
-) -> Vec<Value> {
-    vec![]
-}
-
 #[derive(Debug)]
 struct WhichArgs {
     applications: Vec<Spanned<String>>,