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,