Revert "Refactor external command (#6083)" (#6116)

This reverts commit 0646f1118c.

crates/nu-protocol/src/engine/command.rs

@@ -37,16 +37,6 @@ pub trait Command: Send + Sync + CommandClone {
         false
     }
 
-    // This is an enhanced method to determine if a command is custom command or not
-    // since extern "foo" [] and def "foo" [] behaves differently
-    fn is_custom_command(&self) -> bool {
-        if self.get_block_id().is_some() {
-            true
-        } else {
-            self.is_known_external()
-        }
-    }
-
     // Is a sub command
     fn is_sub(&self) -> bool {
         self.name().contains(' ')