informationpanel: don't change color of scrollarea's viewport

This code was added a long time ago in b1c9b512, but the commit
message doesn't explain why. If we remove it nothing seems to change and
we fix the ugly background on the scrollbar when Breeze draws a frame
around dockable panels.

BUG: 366203
FIXED-IN: 16.12.2
REVIEW: 129861