Remove dead rustboot code.

src/rt/rust.cpp

@@ -103,7 +103,7 @@ rust_start(uintptr_t main_fn, rust_crate const *crate, int argc,
 
     uintptr_t main_args[4] = {0, 0, 0, (uintptr_t)args->args};
     dom->root_task->start(crate->get_exit_task_glue(),
-                          crate->abi_tag, main_fn,
+                          main_fn,
                           (uintptr_t)&main_args, sizeof(main_args));
 
     int ret = dom->start_main_loop();