kcms/lookandfeel: Don't set a fallback window placement
Because m_applyWindowPlacement is set to true, this code was always
getting evaluated, and because a fallback value was set, the net result
was that setting a global theme always reset your window placement to
"Smart" even if the Global Theme that was applied did not set the window
placement mode.
This commit fixes that by removing the fallback, causing the value passed to setWindowPlacement() to be empty, triggering its early return condition.
This is similar to !492 (diffs), but for something else and fixed in a simpler way, because the problem is less complicated here.