Remove is_tail field from Call node

The `is_tail` field on the `BoundKind::Call` node was an intermediate
representation for tail-call optimization and is no longer needed as a
distinct field. The TCO pass now embeds this information directly into
the `RuntimeMetadata` of the `ExecNode`, which is the VM's internal AST.
This simplifies the `BoundKind::Call` structure and removes redundant
information.

src/ast/compiler/binder.rs

@@ -233,8 +233,7 @@ impl Binder {
 
                 Ok(self.make_node(node.identity.clone(), BoundKind::Call { 
                     callee: Box::new(callee), 
-                    args: Box::new(args),
-                    is_tail: false 
+                    args: Box::new(args) 
                 }))
             },