Material: Drop "attached property enforcement"

As can be easily demonstrated, attached properties are getting created
on any Qt object any time it is accessed in any way in QML, be it
property read or write, or even simply querying the object itself like
`delegate.Layout` or `this.Material`. Thus, no such "enforcement" is
needed.

The output is identical when running the following snippet with both
Qt 5.15 and 6.5 QML runtimes.

```qml
import QtQuick 2.15
import QtQuick.Controls.Material 2.0
import org.kde.kirigami 2.20 as Kirigami

Item {
    Kirigami.BasicThemeDefinition {
        id: hack

        Material.elevation: 2

        textColor: hack.Material.foreground
    }

    Kirigami.BasicThemeDefinition {
        id: nohack

        textColor: nohack.Material.foreground
    }

    Kirigami.BasicThemeDefinition {
        id: nothing

        textColor: "#111111"
    }

    Component.onCompleted: {
        print(hack, hack.Material)
        print(hack.textColor, hack.Material.foreground)
        print(hack.highlightColor, hack.Material.accent)

        print(nohack, nohack.Material)
        print(nohack.textColor, nohack.Material.foreground)
        print(nohack.highlightColor, nohack.Material.accent)

        print(nothing, nothing.Material)
        print(nothing.textColor, nothing.Material.foreground)
        print(nothing.highlightColor, nothing.Material.accent)
    }
}
```