KPluginSelector: Remove unnecessary background rect in About sheet
It worsens text contrast and causes visual bugs because it's being set as the background of the entire Sheet rather than its content item, which makes it hide the sheet's nice rounded corners. We could fix that by setting it on the content item, removing internal padding from the Sheet itself, and then manually re-adding it in the content item, but that would be a lot of code to preserve something that has less than ideal text contrast anyway, so let's just kill two birds with one stone by removing the background color rectangle entirely.
BUG: 457648 FIXED-IN: 5.98
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