Remove unused sizeHint() from GridComicsView and ComicsViewTransition

QWidget::sizeHint() is const-qualified, so Clang warns that non-const
sizeHint() member functions merely hide the virtual function of the base
class.

664dac3401 and
9f53ae6efc introduced these member
functions in 2014 without const qualifiers. QWidget::sizeHint() was
const-qualified even in Qt 3. Since these member functions have never
had any effect, they should be removed rather than const-qualified to
preserve the long-standing behaviors of the two classes.

Add a TODO for a similar but less straightforward issue with
PropertiesDialog::sizeHint().

YACReaderLibrary/grid_comics_view.cpp

@@ -469,11 +469,6 @@ void GridComicsView::resetScroll()
     QMetaObject::invokeMethod(scrollView, "scrollToOrigin");
 }
 
-QSize GridComicsView::sizeHint()
-{
-    return QSize(1280, 768);
-}
-
 QByteArray GridComicsView::getMimeDataFromSelection()
 {
     QByteArray data;