Make list-remove look like a red X
Currently its icon is a minus sign. This looks okay when paired with a plus sign icon nearby, but suffers visually in other contexts and when on a flat button. In these cases it struggles to look like an icon, and can seem like a piece of punctuation or a visual glitch.
Instead, let's make this icon look like a red X. That's what the HIG says we should use for icons that remove abstract content, too. See https://develop.kde.org/hig/icons/#icons-for-destructive-actions.
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