ProblemNavigationContext: drop frames of tables with solutions

Not sure the frames around table and cells of solutions where drawn in the Qt5 version, but they have been now, and they look not that great IMHO. And some screenshot I found hints Qt before Qt6 indeed did not process those border style data -> https://userbase.kde.org/images.userbase/e/e3/Kdevelop-bus6.png

This patch removes the style options, present since the code was added in 9139de66 (perhaps part of some unpolished code copy, as the hard-coded color argument hints to some more complexity planned/tried).