QRollEffect: Remove assignments that have no effect

Value of the member variable `done` will be overwritten after the end of
each `if` block.

Pointed out by PVS-Studio static analysis tool:
https://habr.com/ru/company/pvs-studio/blog/542760

Change-Id: Icaf965aaebfa8a238dd3569689e1496a30d4b6d1
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

src/widgets/widgets/qeffects.cpp

@@ -499,14 +499,12 @@ void QRollEffect::scroll()
                 + (2 * totalWidth * (elapsed%duration) + duration)
                 / (2 * duration);
             // equiv. to int((totalWidth*elapsed) / duration + 0.5)
-            done = (currentWidth >= totalWidth);
         }
         if (currentHeight != totalHeight) {
             currentHeight = totalHeight * (elapsed/duration)
                 + (2 * totalHeight * (elapsed%duration) + duration)
                 / (2 * duration);
             // equiv. to int((totalHeight*elapsed) / duration + 0.5)
-            done = (currentHeight >= totalHeight);
         }
         done = (currentHeight >= totalHeight) &&
                (currentWidth >= totalWidth);