🍒 kcms/rules: Make Comboboxes bordered again

The property flat: truewas set since the QML redesign and remained unnoticed because it didn't actually work.

Now that the qqc2-desktop style has been fixed and correctly draws flat (non-bordered) comboboxes let's remove it for a nicer look (just the same as the have had all this time)

(cherry picked from commit 6082de13)