Revert "use again the palette coming from Kirigami.Theme"

This reverts commit 03da9da5.

It doesn't work in Qt6, because they broke import versions compatibility
for the palette property in, which was uplifted to QQuickItem and
marked as "since 6.0", even though it was available in controls since
QQC 2.3 (Qt 5.10). Simply removing import versions does not help
either, as the whole thing just crashes deep inside QtQuick runtime,
which is because they changed palette type from QPalette to QtQuick's
own private thing, and they somehow fail to covert between each other.

CCBUG:465054