[kcms/notifications] Use Kirigami.BasicListItem instead of custom delegate

There's no need since the custom delegate was manufacturing the exact same look that a Kirigami.BasicListitem can provide with the hidden separator setting. However this commit does not use that setting, so as a result the separators now become visible. This makes the list consistent with other lists in QML-based software.

My sense is that the reason why we hid the lines in the past was because the lines had a very heavy appearance, but that was improved recently to look much more lightweight.

Therefore, let's turn the lines on. If we still think the lines are too heavy-looking, we should continue to improve that in Kirigami itself.