katesessionmanager: Do not create QStringLiteral for config group write

We have a `const char*` overload and the QString overload only calls this one.
Consequently, creating a QStringLiteral is not needed.

```cpp
void KConfigGroup::writeEntry(const QString &key, const QString &value, WriteConfigFlags flags)
{
    writeEntry(key.toUtf8().constData(), value, flags);
}
```