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.h

@@ -31,7 +31,6 @@ public:
     void toNormal() override;
     void updateConfig(QSettings *settings) override;
     void enableFilterMode(bool enabled) override;
-    QSize sizeHint();
     QByteArray getMimeDataFromSelection();
     void updateCurrentComicView() override;
     void focusComicsNavigation(Qt::FocusReason reason) override;