rt: Stop calling prepare_valgrind_stack when it's not needed

src/rt/rust_task.cpp

@@ -602,7 +602,6 @@ rust_task::new_stack(size_t requested_sz) {
     LOGPTR(thread, "stk end", new_stk->end);
 
     stk = new_stk;
-    prepare_valgrind_stack(stk);
     total_stack_sz += user_stack_size(new_stk);
 }
 

src/rt/rust_task_thread.cpp

@@ -388,7 +388,6 @@ rust_task_thread::prepare_c_stack(rust_task *task) {
     I(this, !extra_c_stack);
     if (!cached_c_stack && !task->have_c_stack()) {
         cached_c_stack = create_stack(kernel->region(), C_STACK_SIZE);
-        prepare_valgrind_stack(cached_c_stack);
     }
 }