Remove useless mail event with entire that counts entire box as new after openBox

```javascript
var Imap = require('map');
var imap = new Imap(opts);

imap.once('ready', function() {
    imap.openBox('INBOX, true, function(err, box) {
        console.log('Got inbox', box.messages.total);
    });
});

imap.on('mail', function(new) {
    console.log('New mail', new);
});
```

Before this patch the above code would print out
```
Got inbox 5052
New mail 5052
# Then when an actual message came in
New mail 1
```

Now the code above prints
```
Got inbox 5052
# Then when an actual message comes in
New mail 1
```