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/lambda_collector.rs

@@ -57,7 +57,7 @@ impl<'a> LambdaCollector<'a> {
                 self.visit(value);
             }
 
-            BoundKind::Call { callee, args, .. } => {
+            BoundKind::Call { callee, args } => {
                 self.visit(callee);
                 self.visit(args);
             }