kernel: work: Remove meanless foreach for work

Since call this function both in
```C
	k_spinlock_key_t key = k_spin_lock(&lock);

	bool need_flush = work_flush_locked(work, flusher);

	k_spin_unlock(&lock, key);
```

So, there are no flag_get change.

Signed-off-by: Lingao Meng <menglingao@xiaomi.com>