app: remove icon sizing preferences.
With GTK+3, high or low density is taken care by the screen scale factor. Having a separate preferences for this is redundant, and likely even wrong. It may be interesting though to have a new preference later, which would allow smaller scale icon sizing since some people like their icon slightly smaller or bigger, and also when you are using screens whose density is at a limit scale factor. Right now, this can be done through themes, but a GUI settings may be interesting. If I add such feature, let's just do it from scratch later.
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