[Approved] Plasma 6 proposal: remove System Settings Icon view

It doesn't support the "Highlight changed Settings" feature or the "Quick Settings" view, both of which were major features that were well-received by users. It's unmaintained and slowly bit-rotting, like the old tree view was until we removed it. And the very concept of having multiple UI styles for a settings app seems like overkill; I think it would be best if we had just one and made it really good.

@teams/vdg