Revert "Use white background when rendering print preview"

This reverts commit 70f0900799.

It is not necessary any more after fixes to wxDC::Clear() in the
previous commits.

src/common/prntbase.cpp

@@ -2054,7 +2054,6 @@ bool wxPrintPreviewBase::RenderPageIntoBitmap(wxBitmap& bmp, int pageNum)
 {
     wxMemoryDC memoryDC;
     memoryDC.SelectObject(bmp);
-    memoryDC.SetBackground(*wxWHITE_BRUSH);
     memoryDC.Clear();
 
     return RenderPageIntoDC(memoryDC, pageNum);