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/comics_view_transition.h

@@ -8,7 +8,6 @@ class ComicsViewTransition : public QWidget
     Q_OBJECT
 public:
     explicit ComicsViewTransition(QWidget *parent = nullptr);
-    QSize sizeHint();
 
 protected:
     void paintEvent(QPaintEvent *) override;