Remove unused variable to fix build with clang-10

Change-Id: I800df8184b3a713b27c71b6303b98b760a4dfc83
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/gui/painting/qpaintengine_raster.cpp

@@ -151,8 +151,6 @@ void dumpClip(int width, int height, const QClipData *clip);
 // 4 pixels.
 #define int_dim(pos, dim) (int(pos+dim) - int(pos))
 
-static const qreal aliasedCoordinateDelta = 0.5 - 0.015625;
-
 #ifdef Q_OS_WIN
 
 static inline bool winClearTypeFontsEnabled()