Add removeBinding to Google\Cloud\Core\Iam\PolicyBuilder

Something like this would be nice:

```php
use Google\Cloud\Core\Iam\PolicyBuilder;
use Google\Cloud\Storage\StorageClient;

$storage = new StorageClient();
$bucket = $storage->bucket($bucketName);
$iam = $bucket->iam();
$policy = new PolicyBuilder($iam->policy());
$policy->removeBinding($role, [$member]);
```