[effects/slide] Completely delete forced roles

Summary:
The slide effect doesn't completely remove forced blur and background
contrast roles. According to `EffectWindow::setData` implementation,

```lang=cpp
void EffectWindowImpl::setData(int role, const QVariant &data)
{
    if (!data.isNull())
        dataMap[ role ] = data;
    else
        dataMap.remove(role);
    emit effects->windowDataChanged(this, role);
}
```

in order to delete previously set data, we should pass a null variant.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13470