kcm: Standardize device deletion UI

Right now the UI is rather inconsistent, using three different words to
describe the action ("Remove", "Delete," and "Forget") and implementing
a custom dialog plus darkened overlay.

Let's standardize on the word "Forget" and use a standard `Kirigami.PromptDialog`
which was made for exactly this type of thing.

BUG: 456439
FIXED-IN: 5.26