KateBuildView: Remove close button entirely

Kai Uwe Broulikrequested to merge
work/kbroulik/build-and-tabspace into master

When it's just hidden, the tab button will still make room for it. Also happens with Fusion style, so it's not a Breeze bug.

Before

Screenshot_20240816_011106

After

Screenshot_20240816_011126

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