Remove unused themeChanged() from RenderTheme.h

https://bugs.webkit.org/show_bug.cgi?id=270611

Reviewed by Aditya Keerthi.

This patch removes unused function 'themeChanged()' from `RenderTheme.h`.

* Source/WebCore/rendering/RenderTheme.h:

Canonical link: https://commits.webkit.org/275817@main

Source/WebCore/rendering/RenderTheme.h

@@ -138,10 +138,6 @@ class RenderTheme {
     virtual void inflateRectForControlRenderer(const RenderObject&, FloatRect&) { }
     virtual void adjustRepaintRect(const RenderBox&, FloatRect&) { }
 
-    // This method is called whenever the theme changes on the system in order to flush cached resources from the
-    // old theme.
-    virtual void themeChanged() { }
-
     // A method asking if the theme is able to draw the focus ring.
     virtual bool supportsFocusRing(const RenderStyle&) const;